When I was in second grade, our teacher asked us to close our eyes and imagine a pretzel. I closed my eyes and tried to will a pretzel to appear as an image, and I failed miserably. All I saw was darkness and faint squiggles. I didn't understand why so many of my classmates said they could "see" the pretzel with their eyes closed.
Later on, I realized that my mind's eye doesn't actually work like that. It's not like turning on a projector screen against one's closed eyelids and magically seeing things that aren't there. The imagery exists on another plane, such that I can visualize an apple with my eyes open or closed, because what I am "seeing" has nothing to do with my eyes themselves.
Which makes me wonder whether people who think they have aphantasia might not be missing a mind's eye, but rather are unaware of how to active it.