I'd have to know which sorts of realism are intended to comment on whether I think it works or not.
The authentic is usually what I think is worthwhile to explore with realism in games versus realism in the sense of being literal to real thing. Authenticity here meaning its informed by the real thing and models its logic, but doesn't worry about exaggerations for the sake of entertainment.
In video game terms its like how Kerbal Space Program is authentic to real spaceflight and rocketry, but isn't actually anything like it.
In practice I'd be wary of the 'D&D syndrome' where fiction is largely elided in favor of simple mechanical description (IE "I swing at him; A 14! That's a hit, he takes 9 damage.") which I find is not really evocative of much of anything except TT Wargames.
In practice I hate the word elide and find it highly disturbing it has such a vogue in only tabletop spaces. But thats just me indulging my primal instincts rather than anything to do with what you're saying here.
Both systems here are definitely designed with the kind of people who really enjoy describing how they fight in mind, and with them its at its peak, but even without, the nature of the different options makes the less interested at least able to describe their fighting mechanically without it looking as you suggested. Someone who engages purely on a mechanical level has to do so narratively and can't really avoid it unless they just stay silent and don't talk through what they're doing, at which point it wouldn't matter how they engage the system anyway.
I am not real sure why that term, Composure, is being used. Dead guy #1 got run through after each opponent spent quite a while making various moves. Presumably he was a bit slower and/or less practiced than his opponent. Guy number 2 doesn't seem dead, but he's got a knife in him and perhaps was overcome with pain/shock/blood loss. He could well be fatally injured. #3 simply failed a morale check or in any case he appeared to be more of a lackey of guy #2 who didn't really have an interest in fighting.
Composure is being used to combine and abstract a number of things, but the key thing it models is the pacing of a fight, much more deliberately than HP does in most implementations.
If we broke down that fight into the equivalent game mechanics, the flirst person who got stabbed in the neck would have lost his Composure (aka, dropped to zero), and this gave his opponent the easy Killing Blow. Meanwhile, the second guy who rushed in isn't dead; he would have taken a severe Wound, and this in tandem with their short exhange likely dropped him to zero too. But he gets finished off with a kick to the head, an Incapacitation in game terms. The last guy, as you noted, chucks his sword, sees the attack wasted, and bails. Our protagonist fighter meanwhile had his Composure brought fairly low, and he took at least one minor Wound, but otherwise lives to fight on.
I don't think RPG combat systems really model this well, and the inherent reason for that is simple: most traddish RPGs seem fixated on a model in which the player always has the say on what options her character chooses next, but reality often, perhaps mostly, doesn't work that way.
Well, this partly the value of input randomness in the system; when you make your Combat Roll, you're pre-rolling your Actions for the Round. This not only makes for a more interesting tactical experience, but is also a great stopping mechanism for just repeating the same actions over and over. Which when combined with Output Randomness of Damage/Defense and the secondary layer of Input Randomness in Momentum, increases the tactical depth considerably.
That then gets combined with Passback Initiative, tying popcorn style Initiative to Reactions, which unless one side completely dominates the other in Skill and dice rolls, results in a strong, punchy, back and forth feel.
How is this different from the modeling in a melee round in say 4e (or 5e, etc.)? Now, if you say that Labyrinthian DICTATES that fiction must be described and specified in order for the situation to be resolved, or further resolved, that would be one answer. D&D doesn't dictate that we determine what exactly rolling a 2 on an attack roll means, fictionally. Dungeon World OTOH absolutely does! You cannot process moves in DW absent fiction, and moves are triggered in reference to the fiction, affect the fiction, and then feed back into further move descriptions (with mechanics constraining what are allowable fictions, etc.).
I related a lot of this above, but the system doesn't really need to dictate anything, as it lends itself so well to people doing it anyway, even if they're not strictly interested in doing so. Interpreting a Clash is as much up to players adding their own flare to the fiction as it is to speaking to the mechanics you used, which are themed to be 1:1 with the fiction anyway.
This especially becomes poignant given the ability to Improvise new actions and improvise new uses for mechanically defined ones is available for literally everything. If what you think of is in-line with the tone of the game your group wants to go with, then you can do it, no questions asked. (This is why 15 Spells is all thats needed to depict the full capabilities of 3 different magic systems, and arguably then some)
As for the difference from 4e/5e, a lot of it is rooted in ludomechanical-narrative harmony. In otherwords, its designed to resolve common problems mechanically (like yoyo healing, dominant strategies, the meat points issue, and others), but in a way that aesthetically themes those resolutions to be in-line with the narrative of whats being depicted through them.
I think, given my perspective rooted in 4e D&D play, that 'yo-yo healing' is only an issue if you insist on considering hit points as being some sort of 'meat' or similar. I've always considered them to be largely just an abstract measure of overall durability, luck, and resolve. I also grant that 4e has some of the negatives of other D&Ds in the sense that it really doesn't ever contemplate 'loss of control'. It is also designed to cater to the potential for a very fictionally disconnected kind of play, although that leaves a lot of the more interesting aspects of the game in the ashcan (and I can only presume exists due to a fear by WotC that a more explicitly Narrativist design would get too much pushback).
Yoyo healing as its typically used refers to the emergent gameplay that results when there's no meaningful consequence to losing HP, so it isn't bothered to be restored, as its more efficient to wait until the person is either downed or is so low that minimal hits could down them. This problem gets exacerbated by bloated HP values, as trying to maintain high HP across a party becomes very costly within the balance of games with the issue.
Composure Saves, in tandem with heavily condensed CP values, resolve this issue. Its incentivized to maintain high Composure, and the low values mean its easier to justify keeping an entire party upkept on this, as even if a particular method of restoration is high cost, its going to be considerably more effective.
there's really no such thing as the equivalent of a 5e combat round where nothing happen
The same can be said here. Keep in mind the balance here; this is a game where you could be throwing out 6d12 in damage every Round when the equivalent HP mechanic for all creatures can be no more than 150, which can be bypassed through a parallel lethality system. So regardless of the mechanical systems involved, fights are designed to not be a drag.
This is why the basics page identified three different kinds of fights. Scraps in GunFu, Bouts in Labyrinthian, are explicitly used to handle low stakes combat, which in mechanical terms you're basically fighting one to a handful, at most, of what are basically Mooks. Enemies so weak a single combatant can take them on within the span of a Dungeon Turn, with zero need to shift the entire party into the combat procedure for Shootouts/Skirmishes (for your typical RPG fights), or Operations/Battles, for Mass Combat.
But even within these latter two, the distribution of Mook like enemies is important. For one, because in both systems we're looking at a power fantasy, and being able to put down even a lot of enemies quickly plays into that, but for two because much of the time fights at these scales come with a requisite increase in the stakes. As such, not only is more time deliberately given over to them because the stakes are suitably high, but the enemies involved get more robust. Mooks stick around a little longer, and dedicated "Boss" enemies are usually in the fray, designed to require specific tactics to defeat; or, at least, more thought than just plowing through them with brute force.
This handy dandy scene from DNDHAT is a great example of just this very thing, and if it wasn't so awesome I'd be extra peeved at how many of my ideas ended up being expressed in this fight despite them predating the movie, and how it ended tracking with something I didn't come up with until I started on GunFu.
If we translated this into Labyrinthian terms, this was a 4 Round Skirmish. The first round against the Mooks, another up until the main Thayn is tossed into the wall, another as they shift up the stairs and Xenks sword is dropped to his dagger, and then the last one starting as Xenk has his dagger raised before they go in.
The Mooks all die as part of his 2 Actions, well within line with what an equivalently high level character could do, and that just leaves the main guy who takes a further three rounds to take down, as he's relatively evenly matched.
And we even see a great, if unintentional, depiction of what the parallel Lethality looks like, which in Labyrinthian is through Wounds. All characters can take up to d8 Wounds before they're open to Killing Blows, and a combination of Armor and high Skill can mitigate this. Armor can absorb Wounds outright at a cost of Durability, and with a high Conditioning Skill, players can push their lethality threshold up to d12 Wounds. Enemies meanwhile can actually reach d20, which means they can't be killed through Wounding at all. Enemies in general are tied up in a Difficulty Die, which defines the Wounds they can be killed at and provides a nice, variable scaling difficulty. Die is rerolled every round and adds to everything the enemy does.
And funnily enough, due to the nature of Daggers in the game, how he defeats the guy completely tracks with how Wound Lethality works. Daggers can't use dice higher than a d6, and if he dropped to d4s, that would easily explain all the stabs and cuts he gets in on the guy as he'd have an easy time across 2 Actions and 6d4 to generate enough Momentum to do so, utilizing "Strike" to inflict and escalate his Wounds, before he can finally take the killing blow on him.
For clarity, I uploaded the basics page for Labyrinthian as it stands atm, still missing the Composure save details because I haven't gone back in to finish revising the doc. The key difference between it and GunFu, aside from the different setting and combat contexts, is in the nature of Lethality.
In GunFu, it revolves around breaking Reactions to get a lethal hit in, modelling accuracy amongst other things in a very streamlined way, with Composure damage as a secondary means. In Labyrinthian, lethality is also accessed through Composure Damage, but rather than Reactions, its based on Wounds as a primary means, which tracks better with the fantasy sword and sorcery feel.
