But that takes this back to what I thought Ovinomancer was saying in the first place, that D&D can't be played in an improvisational manner, and that's clearly false. So there has to be more to it than that. Again, I accept that there are "No Myth" games that lack what I think Ovinomancer means by "curation", based on examples of play I've seen, but I'm not at all convinced FATE is an example of one of those games.
Yes, it is clearly false, and I haven't said you can't improvise in D&D. I said that you can't not curate.
So, let me disimpact the difference between improv and curation. Curation is the creation of story elements that are revealed to the players during play. If you decide that this NPC, which you just created improvisationally, cannot be bribed because they're too loyal to the king, then this is curation of story. This is the secret part of backstory, which can be created on the fly, that is revealed to players as they navigate the fiction the DM is weaving.
Curation is what's required for games where the players are expected to discover, navigation, and be challenged by the GM's story elements. Fundamentally, it's the difference between the GM deciding how hard this guard is to bribe do to story factors the GM has created and the outcome of the players' action resolution determining if the guard was bribeable.
The classic secret door example springs to mind. In a curated game, the result of a check for a secret door calls up the GM's prep notes to see if a secret door was placed there, and then if the roll was sufficient to find it. In a non-curated game, the player makes a check and, on a success, there's a secret door here. This works because the existence of the secret door doesn't invalidate future plans because there were no future plans.
If you recognize this example, you'll know that this example contrasts very valid ways to play, neither of which is better -- just different. So, curation is not a negative thing -- it's important that the GM in curated games be able to present the fiction according to their thinking of how things are, because these games are built on the concept that play is fun when players navigate and explore the GM's fiction.
I'm a high prep DM, but for any town bigger than a hamlet I've never been able to set up the town in advance to a degree that means I'm not improvising most of the places and interaction in the town. I don't know every bar in advance, much less what is in the bar and who runs it. Most of that tends to be made up in play based on player cues. I do have creative input into it but so far as I can tell that doesn't differentiate D&D from FATE or PtbA or anything else.
Resolutions in FATE and PbtA alter the fiction according to the player, without GM veto ability. That the GM narrates outcomes in D&D is part of the curation -- the GM sets the success and failure conditions, the difficulty of the target, and the result of outcomes according to how the GM thinks fits these support the current play. FATE and PbtA do not have this strong GM override (they have weaker mechanisms, but aren't fully Story Now in RAW form).
Ironically, this is even more true of FATE or PbtA than D&D, which typically defines the stakes of a skill check as pass/fail explicitly so that the DM has very little interpretation to do in terms of what the fortune check meant. Where as FATE explicitly empowers the GM to decide whether to give partial failure, success with a setback, and a variety of other techniques and explicitly encourages the GM to use their own judgment as to what would make a good story to narrate what the fortune roll actually meant. So while you may be right that there is a certain amount of GM fiat involved in D&D's resolution, to the extent that there is, there is vastly more in FATE. And while the FATE GM could encourage the player to narrate the consequences of the check, and some do, it's not required by the system and you can in D&D encourage players to narrate the consequences of their failures as well - and some DM's do.
Even if I accepted everything you said in your argument, it still would not prove that there is a categorical difference between D&D and FATE baked into the system. I'm not saying conclusively that there isn't, but you haven't really addressed that.
Again, the difference is that players have authorities in FATE to control and change the fiction. In D&D, they may only request changes to the fiction, pending GM approval. This is why I use the term 'curation' for D&D -- the GM must approve any changes to the fiction, and should reject changes that will be a detriment to play. What constitutes a detriment doesn't mean make play bad in and of themselves -- things that invalidate large amount of prep and story are a detriment to play sometimes in D&D and need to be avoided. This is a net good for D&D style play, although it won't sit well with everyone (nor should it). The success and popularity of D&D speak to the fact that this isn't a bad or poor way to play the game. And, there's lots of room in curation for players to even have a sense of ownership over parts of the fiction. My play is very lenient to player input and even player creation of fiction (talking outside of character backstory here), but it's a given that I, as GM, always have veto rights. In my Blades in the Dark game, I do not have veto rights as the GM -- the mechanics speak and determines who gets the say.
Let me compare and contrast how a bribery attempt works in 5e versus Blades. Hopefully this will showcase curation. First, let's say both scenarios show up unexpectedly, and the GM is doing some quick ad-lib. A roguish sort of PC has been caught up in a compromising position by a guard, and is offering a sizable bribe to escape custody.
In 5e, the DM will consider how the guards in this area fit into his campaign -- is it largely law and order, or is it more corrupt. Then, the GM will consider how this guard thinks, and compare to the amount offered This will set the DC for the attempt (or if the attempt is even allowed). A roll is called for, and if the PC succeeds, the guard accepts the bribe, if the PC fails, the guard refuses the bribe.
In Blades, the GM considers the situation - is the PC acting in a controlled situation, a normal one, or a is the PC desperate. Given that the PC just got caught, the GM will likely say this is a desperate situation. Next, the GM looks at the guard's tier (set by setting, but the Crew knows this because they got to pick the target). This compared to the PC's tier will set the effect level -- let's assume normal (even). Then the PC says what attribute their rolling (they get to pick) and a roll is made. On a success, this guard is bribeable, and the transaction occurs. On a partial, the situation is good and bad -- the guard may be bribeable but haggle for a higher bribe. On a failure, the guard isn't bribeable, or another guard shows up, or the PC accidentally insults the guard's mom -- something bad happens.
Here, in the 5e example, the DM considers their secret backstory to make determinations. In the Blades example, it's purely on the current fictional state and the known stats that determine the possible check results, but the actual check is a fixed pass/partial/fail set and the player gets to set the attribute used. Also, the player has options to increase dice rolled, improve effect, and mitigate failure, so even after the situation is set, the player can add to the fiction to improve or even change outcomes. The GM has no veto authorities for these, no counter-balancing authority. The result of the player's action determines the next state of fiction and play proceeds from there.
Functionally, FATE should play like this. The advice for FATE is spotty and poor and, in my opinion, actively fights how it should be played. It recommends that the GM should plan some things, but isn't clear that this should stop at conflict setup/world gen and allow play to generate organically by following the fiction created in play. Trying to curate a FATE game leads to bad outcomes, which pretty much is what I've seen in every example of people complaining about a bad FATE game.