This is what I used to think. That "aphantasia" was just people arguing semantics about subjective experience. But my belief now is that if you think this way, you likely have aphantasia (I think I do).

A couple things that convinced me that the phenomenon is probably real:

1. I can visualize when I dream, or when I'm dozing/napping. But I can't while awake

2. I can "visualize" sounds, so I can hear a sound/song in my head and it's pretty much like actually hearing it. But I can't do this for imagery

Admittedly, neither of those is a smoking gun and I'd also be interested in a more objective test.

This is my experience too. I have aphantasia for all intents and purposes but I have had one single experience years ago during meditation where I suddenly experienced "being in" a scene that was vivid and clear that I knew to not be real. It's nothing like when I "imagine" something and don't see anything.

It was also nothing like when I dream, where I too can visualise - it was much clearer.

It's clear to me that most of the time when people question that this is possible, it's an indicator they have aphantasia.