It doesn't model the real world, or any believable hypothetical world. It only models narrative contrivance, which is entirely at odds with that.
The AD&D healing rules actually provide a useful model, within the subset of circumstances which are likely to occur during gameplay. That you want it to model things it has no reason to model is a problem of the mismatch between your desires and what it does.
First, it's good to know you can't manage suspension of disbelief with a Marvel movie. That, however, is your issue and not the issue of the millions of people who show up to watch their films.
Second the AD&D hit point and healing rules do not model
any believable hypothetical world that has any sort of narrative consistency. The healing is ridiculous, as is the hit point model. Indeed it's pure video game of the sort that's not even attempting to pretend to be realistic. No one in a remotely realistic world can take a full strength hit from an orc with an axe and recover in any reasonable time scale, and in any sort of realistic (or even decent larger than life) world fatigue is a thing.
An almost untiring robot who can take a full strength hit from an orc with an axe and who has the same offensive skill pretty much whatever they've done previously is a playable character in a beat-em-up, not a reasonable approximation of a human being.
You've missed the third possibility, which is that armor is much more effective than we'd expect. Given that low-level characters do die when an orc hits them,
Not always. A second level thief who rolled slightly above average on hit points or has a Constitution of 15 and had average rolls will still be standing and hitting back just as effectively when the orc hits them and does maximum damage in AD&D. Second level is pretty low level, leather armour is pretty poor armour, a thief is not magic, and this problem is therefore baked in to the system.
The rules for armour are explicit within the game - they change your armour class. It's also possible to rust-monster armour and do other things to degrade it, which somehow does nothing to hit points. If neither changing armour for a padded doublet nor explicitly weakening the armour does anything to hit points, and neither does replacing explicitly mundane armour with magic armour then we can conclude fairly definitively tha tthe suggestion that hit points are a consequence of armour does not match the game rules.
(While a high-level character would still have their HP if they took their armor off, such an occurance is outside the basic assumptions of gameplay, and not worth modeling.)
Your games may be boring enough that you never have scenes at court or weapons and armour checks. In mine it's rare but not unknown, especially when a character specialises in non-magical sneaky stuff. It's also not unknown for assassins to catch the PCs in the bath house because they know that in a believable world people have to take their armour off. And remember that by "high level character" we mean "at least 1 in 5 first level fighters".
Also numerous modules have started off with the PCs in jail or captured by slavers. This is therefore well within the expected range of gameplay.
I agree it would be outside the basic assumptions of gameplay for a beat-em-up video game or a tabletop wargame. But the very fact you have to restrict your game to such a limited gameplay model shows that you are not trying to model any believable hypothetical world, but only to model sufficient narrative contrivance as to provide a figleaf for a game.
Also that you talk about "basic assumptions of gameplay" is telling - in that it is full acknowledgement that AD&D is making no attempt to model a believable hypothetical world so much as it is to be a game that runs under the rules of a game and where the world is mostly a backdrop. There's nothing wrong with that of course as long as you don't pretend it's realistic in any way.
Classes which lack armor proficiency do not represent normal people, and their inherent supernatural abilities can account for the same discrepancy.
Once more for those in the back
the thief has armour proficiency but normally wears light armour. They are also pretty short of inherent supernatural abilities.
As to whether or not the assumption is fair, well... the name of the game is Dungeons & Dragons. That says something about the intended mode of play.
It's a fighting game where you beat your way through dungeons by walking forward, mashing A, and taking down opposing health bars while being at full capability until yours drops - in the same way you do in Double Dragon or Streets of Rage? And where things that can't be handled in this manner are outside the basic assumptions of gameplay and therefore do not happen?
If that's the case then the problem isn't that 4e doesn't model the world. It's that
because it models a decently believable hypothetical world it forces you to confront head on just how unrealistic AD&D has always been. 4e is merely the messenger. Now it's possibly a decent critique to say it's in the uncanny valley of attempting to be realistic and falling short as compared to AD&D's pure cartoonishness that doesn't even try for realism.
