https://news.ycombinator.com/item?id=9280134
Notably, ~gwillen there pointed out Francis Galton's 1880 study of the matter – where he started with the idea that mental imagery was a hoax, as he himself did not experience it. His surveys convinced him that it was instead common, and that only ~3% of people lack it entirely – very close to the 2% estimate Ross passes along from a 2009 study.
So: still surprising people, but known for quite a while.