For what it's worth, I spoke to my friend who professes to be able to visualise things very vividly. I think there is something to it because she was able to confirm that she can literally see the thing she is visualising as if it's there, I asked her many questions about it so I don't think there is much room for miscommunication. She confirmed that it is as if that thing is there and that she can hold that imaginary thing in her field of vision for long periods of time. She can do this with her eyes open.

There must be some sort of spectrum of ability with this stuff. But I agree that it's difficult to prove and measure. I do think that great artists must be on the more "gifted" side of this spectrum.