Being "made up by the GM" is absolutely not the same as "constantly and secretly changing whenever the DM feels like it." With illusionism, you must be committed to denying the players the chance to see that the world is being made up on the spot. If you're open about that (which I am, in the exceedingly rare cases where "re-frame things to be where they need to be" is absolutely necessary), then it's not illusionism, because you're actually informing the players about what's going on.
When I GM, I make up the world on the spot all the time!
This has to be done to frame situations, in games (like BW, or like Classic Traveller as I GM it) that don't rely on prep of situations in advance. Even in prep-oriented games (eg in Prince Valiant I tend to use scenarios from the rulebook or the Episode Book), it's often necessary to add bits of detail to framing that aren't present in the (pre-)authored material.
And making stuff up is also pretty central to the narration of failure, as per my post just upthread.
if what you really mean is stuff like "glossing over the 17 branches off the road they could have taken, because they're heading for the Fire Swamp and thus don't really care that they could potentially go elsewhere," okay, that's fair. I just...wouldn't call that "illusionism" anymore, you're just glossing over unimportant details and false starts so that the party can focus on the things they've already chosen to do. As far as I'm concerned, you're defending people presenting each and every one of those 17 branch points as an Actual Serious Choice that the party must think about....only for literally none of them to matter one bit, despite spending table time on making them.
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If the ogre shows up literally no matter what you do, literally no matter where you go, literally regardless of choices or circumstances, isn't that the same as having "the way forward" (the right choice) definitely never happen on the first two tries? Because both of those things are "event X happens, literally no matter how you choose to behave," just "event X" is "you fail twice and then succeed" vs. "an ogre appears."
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choices, even small choices, should in general have merit. Now, maybe you have a reasonably well-established reason why SOME ogre will show up whichever path the players take, because this is ogre country. In that case, it's not that the players' choices don't matter, it's that some previous choice(s)* mattered for determining whether they might encounter ogres, e.g. "we decided to adventure in the Wood of the Western Wyld instead of the Southern Sirensong Sea." Or maybe it really is the same singular ogre, but the choice the players make affects when or how they encounter this ogre--because he's tailing them (again, presumptively due to past choices*), or both the left and right routes go through places "in his territory," but he starts on one side before going to the other, meaning the choice might mean starting off on more positive footing (meeting him outside one of "his places") vs more negative footing (running into him AFTER looting one of "his places.")
So...yeah. I'm sure players expect encounters. But unless there's a good reason for ogres to be generically about (an easy thing to establish, mind!), or some other difference occurs as a result of the players choosing path A over path B, I do think it's in the same wheelhouse as the non-fudging examples you described. Same as changing midway through a murder mystery who the real murderer was, or deciding that the party would definitely encounter the Countess ten minutes after starting down either the left or right path. If the choice isn't really a choice, just gloss over it; don't create fictitious choices that appear to have value but are literally irrelevant.
I'm not 100% sure what you mean by "choice" here. Do you mean a choice made by the PCs, or by the players? And if the latter, do you mean an action declaration, or just a bit of colour narration?
If a player narrates that his/her PC wears a headscarf, should that matter to play? Maybe in The Dying Earth, which makes quite a big thing out of fashion, and hats in particular; but often I think it won't and that's probably OK. It's just colour.
If the players are explaining that their PCs are trekking across Europe, and they explain how they are travelling north of the Alps, or south of the Alps, should that matter to play? Or is it just colour? That will depend a fair bit on the details of the game being played; most of the time I will tend to prefer a game where it is just colour.
As far as the "quantum ogre" is concerned, is it important to the game that the players be able to exercise control over scene-framing? If the game is B/X D&D, then the answer is
Yes. If the game is Prince Valiant, then the answer is mostly
No. That's not to say that Prince Valiant encourages illusionistic GMing - quite the opposite. It just means that it treats travel and geography, and especially the minutiae of travel and geography, primarily as colour.
EDIT: You mention the ogre tailing the PCs, or seeking retribution against thieves. It seems to me that, in a non-railroaded game, those would normally be states of affairs established by the GM as consequences of failure. Or perhaps if the game is stalling a bit and everyone looks to the GM to see what happens now, the GM might introduce hints of such possibilities to get things moving (a "soft move" in PbtA parlance).
Treating those as "given" elements in the fiction that can then be used to adjudicate action declarations, rather than as a resource for the narration of failure or for framing, strikes me as tending towards railroading.