I don't see any benefit to doing this - appear to offer a choice, but then negate that. Why have apparent branching paths with the ogre down whichever one is taken? Why not a single linear path with the ogre on it?
Illusion of choice. The idea behind illusionism is appear to offer the players agency without actually doing it.
I had known about the technique for a long time, but I first really started thinking about it three or four years ago when I was wanting to run a CoC adventure path ("Masks" is it happens) and I went to an oldbie CoC forum to get advice from long time CoC GMs about how to run such a campaign. And basically, all the advice I got about the system and the adventure was "just slather everything in Illusionism and it will be good".
That was not the advice I was in any way expecting about running CoC, which in my play with it and in my head was not a game with the aesthetic of Illusionism railroading the players through scenarios so that they win (or lose).
I'm against deceiving the players.
In general, I am too.
I think this is different though from standard tropes like "the PCs just in time to stop the evil ritual" - these aren't necessarily deceiving the players.
Well, maybe. I agree it isn't necessarily about deceiving the players, but it is the exact same technique. Remember, I'm all about techniques. If you are using a technique to fudge when the ritual happens so that whenever the PCs arrive, it's just in time to witness and stop the ritual, then that is Illusionism. You are playing with the fact that the players have limited ability to view what is happening outside of their character's knowledge to arrange events in such a way that you think they will be most dramatic. The exact time the ritual occurs is "Sometime in a two week period, when the PC's arrive", which is very much not realistic and not driven by carefully planning and imagining what the NPCs are doing off stage.
So while you may feel more comfortable using Illusionism to achieve these effects because "the heroes are always just in time" is a widely used narrative trope, it's still illusionism. For me, if I apply a definition of Illusionism consistently, what I find is that at some level even the most rigorous campaign is using a lot of illusionism. That's because it's impossible for a GM to actually run the full imagined world in all its details as a true simulation. You are always fudging the reality of the setting to a large extent.* So the question is not so much "use illusionism or not", but when to use illusionism, to what degree, and for what purpose.
*The one argument in this thread I don't want to entertain and get into is, "Since everyone is using illusionism, then there is no such thing as Illusion" or its cousin "Since everyone is using illusionism, there is no difference in degree between what I'm doing and what you are doing." I 'm simply tired of trying to explain that there is a difference between a little and a lot of illusionism in terms of the agency provided to the players and what you as a GM are trying to achieve.
And of course it's fine for NPCs to seek to deceive the PCs. I think a deceitful GM is breaking the table contract, but deceitful NPCs are just doing their thing.
NPCs being deceitful is just part of that realistic animation of the setting. The NPCs are (hopefully) behaving in the way that is reasonable for the circumstances they are in and the personality and goals that they have.