alt Hacker NewsLearning to juggle involves easily observed objective activities that other people can check, and which can be learned that way--you can watch someone else juggling, read book that describes the tossing of the balls, etc.
How do you do any of that with visualizing? If I'm trying to learn to juggle, I can watch someone else juggling and say, I want to do that. How do I watch someone else visualizing and see what to do?
Yes, it's harder to evaluate whether someone can visualize well, but that does nothing to invalidate my point.
Have you actually dedicated significant time to trying to learn how to visualize? I suspect that if you did, you'd realize: oh, wow, maybe I can actually get better at this. If most people who give it a serious effort also come to that conclusion, I feel like we've pretty much settled the question. So it is testable, just not as easily.