I'm going to second you for a different reason:

I guess one thing I don't really get or perhaps don't by about phantasia is that the way our whole visual field works in a big illusion: we can really only see a tiny fraction at the center of our vision with any meaningful detail and we don't even realize each eye has a blindspot.

Thus we can very easily believe we "see" something that we clearly do not see.

To make this into a more concrete test, there's a study that most people don't even remember what a bicycle looks like [1]. I'd ask somebody who insists they have phantasia to picture a bicycle, ask them if they see it clearly and in full detail as though it's in front of them, and then say great, how many gears do you see? Does it have a chain? Which way does the handlebar go? Please draw it.

Of course they don't need to draw a perfect bicycle, they could be imagining an imperfect bicycle, but I suspect what would happen is that they would be dynamically adding detail as you asked them (e.g. about the break cords) rather than having seen the whole thing in all of its detail at once. I think that's just an implausible amount of information to pull up for the average person in a moment and render.

1 - https://twistedsifter.com/2016/04/artist-asks-people-to-draw...