Crimson Longinus
Legend
The whole game is an illusion. An illusion that there is an expansive real fantasy world even though in reality it is just some vague ideas in people's heads.1) If no one is being fooled, how can you call it an illusion then?
Something to be changed it must exist first. And I reject the idea that things that are just some GM's musings 'exist' in this sense. You're confusing preliminary storyboards and deleted scenes to canon. Canon is what happens, canon is what's on screen. Before the players were told about the ogres, the ogres didn't exist anywhere, so they cannot have been changed.2) 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.
I mean, come on man. You know that arbitrary ad-hoc modification of a world is not absolutely identical in all ways to ANY form of inventing an imaginary thing. You're a smart and well-read person, from what I can tell; you've interacted with media enough to be familiar with things like "canon" and the like, which explicitly fork apart arbitrary change to the world from well-grounded change to it. One of these things is okay. The other is not. Don't pretend that illusionism is precisely the same as invention. The former is explicitly, specifically, intentionally hidden from discovery. The latter, in general, is very much intended to be discovered.
Now, 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.
(No, Saavik is not a half-Romulan.)