Wow you certainly didn't pay attention to the previews if your not thinking that is a illusion heck the first preview literally showed a meteor coming down on the flower field.
heck look at time stamp 0:13-0:14 in the first preview it's pretty clear and obvious that it's an illusion.
I saw it, and it showed Ash and co. + Naganadel just standing in a specific footing with the surroundings moving like a vision with a meteor going past them, so I'll agree that it was an illusion.
But in that specific scene (where the meteor falls), the surroundings aren't quite similar to the flowery meadow landscape that is afterwards shown in the preview + in the second preview too. So even though the meteor seems to be an illusion type of thing, that doesn't quite confirm that the flowery meadow landscape is also an illusion.
Also yes there is something that suggest that Naganadel is the one creating it since it's eyes just started glowing rainbow and then the illusion popped up, not to mention that it's coming out spreading out from Naganadel, so it is a illusion no matter how much you try to argue it especially since it's spreading from Naganadel and not Necorzma.
Well, first of all the scene where Naganadel's eyes are glowing rainbow and the scene where the flowery meadow landscape starts to spread could possibly also be 2 different scenes which are being shown in the preview concurrently; we can't be sure that those are the part of the same scene one after another. The flowery meadow landscape seems to be spreading from the point where Naganadel is; but it could be just that flowery meadows full of light is starting from the point where Naganadel is present, that's it, but not necessarily that Naganadrl is creating the flowery meadows as illusions.
Considering Naganadel doesn't have the Illusion ability like Zoroark nor it is a Psychic type who can just share its own memories like visions (like Latias did in M05, so it creating an illusion would pretty much ridiculous and would hardly be logical. If that truly happens simply because of plot convenience and with the weird excuse of weird alien power, it would rather look like a cop-out or something that defies logical explanation IMO, so I'm not sure how that will work out.