The author didn't say he "felt" that most people ride around on incorrectly size frames, he said they did. But let's not argue semantics, and let's take this argument on its face value.

Assuming "most people" ride on incorrectly sized frames (either because they were sold incorrectly, or because they tolerate it), you should be able to go to any group of 100 riders and find more than half of them on bikes they don't fit on.

If you believe that, go for it. I'd rather not condemn a majority of riders for riding a bike that I don't think/feel/assume/believe is wrong for them. As long as people are riding who gives a crap what or why?