The setting fluff is both (a) absolutely essential, as in without it I would NEVER do this no matter how bored the players might seem, and (b) not something I can just change on a dime whenever I want to whatever degree I want. I have actual constraints on what I'm allowed to do. Yes, I can invent new things and proceed to demonstrate them in the fiction. But that's not the same as "the haunted house is EXACTLY wherever the players go, because I've decided that's where the players are going." It takes effort on my part, sometimes a lot of effort, to make these additions (or changes) happen--even in a world following the Dungeon World DM principle, "Draw Maps, Leave Blanks."
Asking for a random encounter roll when the party is choosing to sojourn through non-patrolled land, or saying "you got jumped by nasty monsters" when it is a well-established possibility that nasty monsters are A Thing that really does just randomly attack.
As an example: if the party is looking bored while, say, on a sailing ship in the middle of the ocean? Nope, not gonna spring a random encounter on them, no matter how convenient that would be for me as DM, because there's literally nothing I've done that would establish that as a possibility. I would have to do real, serious work to establish it, and leave some breadcrumbs for the PCs to learn about it, and very specifically give them time to choose to follow up on that if it isn't just stated out in the open. E.g., openly stating it could be the captain of the ship they're on inviting them to a private dinner, regaling the party with tall tales...and then getting more serious and explaining how there are Things that come from the deep, such that the best sailors always carry a cutlass even on routine voyages...and a holy symbol just in case. Leaving breadcrumbs could be mentioning that there's been a sharp increase in demand for mercenaries on trading vessels, or that Waziri mages (who normally avoid the docks) have been spotted dockside, collecting reports from sailors about unusual phenomena. Stuff that's noticeable, and that the party could spend a little time investigating as long as they aren't on a super-tight time budget. That would give me a foundation to build on.
I absolutely still think it is deceptive to use illusionism--which is not the same as "dishonest." "To deceive" is "to mislead by a false appearance or statement," which is exactly what happens when you present a choice as mattering, giving it the false appearance of impact, when it actually has no impact. "Dishonesty" is about lying, cheating, or theft, which doesn't apply to this situation (or at least I can't see how it would). I mean, the top three definitions of "illusion" literally all reference deception or false appearances in some way. I don't see how it's possible to argue that illusionism is not built on giving choices a false appearance of significance where there is none.
It is precisely the "GM-fiat-ish" that is the problem.
And yes, I run DW, I don't just play it. (Well, I don't play it at all right now, I just run it. But I did play it for several years before, and have played some Masks and IIRC one other PbtA game, though I can't remember what it's called.)