Maybe people aren't saying "I can personally hallucinate at will", but many people saying "I can't visualize images" think that other people can hallucinate at will. The twitter poster even says "the way your eyes see?" which is pretty clearly asking if people can hallucinate.

Depending on how someone comes into this discussion, their prior experience, and the particular language they come across, it's easy to interpret these images as representing... well... images. People who say "I'm a 1, I'm a 2, 3, 4" are probably just saying they can think about and recall the shape, texture, color, etc of things, and can't actually see shit. Having a scale of "good image, bad image, outline" was probably meant to be very abstractly tied to this ability to think about those details.

I think if you took a group of completely neurotypical people, whatever that means, who all have EXACTLY the same sensory experience, they would start labeling themselves on every part of this chart and completely misunderstand what everyone else is saying about their perception. You'd have 1's and 5's despite no actual difference.