This is a good theory but it doesn't really match the actual data. Aphantasia is a real thing-- some people cannot create mental images. A good test for this is the "Visualize an apple... what color was it?" test. You are right that language is a major barrier to describing your internal experience, and this likely causes a lot of false positives and negatives, but there are ways to get around that. Also, I've never heard of someone actually confuse mental imagery for a true visual stimulus, and misrepresenting mental visualization in that way is... strange, at minimum.