I understand the claim, what I'm saying is that it would be difficult to confirm. Not having a visual reference would mean their experience of visual input would be distinct from ours, but concluding that it doesn't happen at all is a stretch.

As a hypothesis, my guess would be their experience of visual sensation of a hallucination would come as raw input - flashes of light or something.

The problem would come with asking that person to tell you if they see flashes of light if they've never experienced real light. How their brain re-wired that chunk is a complete mystery to an outside observer. If visual processing is reassigned, it would be to something completely unrelated: like, light perception could now be tied to decision making, with good ideas feeling brighter vs risky ideas feeling dark. Or vice versa.

But to them, that processing of visual feedback isn't a visual experience, so asking if they've seen flashes of light would be like me asking you if you've ever tasted an ethical dilemma or some other concept: the question wouldn't make sense, and we would have no way to make it make sense without knowing ahead of time that it's tied to decision making. And if it was, a visual hallucination could come as making them feel erroneously confident about a risky behavior simply because the visual cortex is giving the perception of brightness to literally every thought.

So again, that would be really hard, if not impossible to assess, and claims to have done so would need a lot of evidence to back it up.