I'm familiar with these studies but I don't think you can assume that activity in the visual cortex is phenomenal experience. I'm sure there are differences in self-reported aphantasics and people who don't claim to be aphantasics, but I don't think the existence of differences proves there is a difference in phenomenal experience.

Prove is a strong word. But it supports the existence of the condition as it coheres with our scientific models of visual experience. You can always reach for obscure philosophical puzzles about phenomenal experience to try to invalidate the science, but at that point you're just having a different debate than the rest of us.