In fairness to those who talk about belief in a flat earth at the time of Columbus, there is I believe a grain of truth to this. While it's undoubtedly true, as the author states, that educated men knew for centuries the shape of the earth, that wasn't necessarily true of the uneducated - among them ordinary sailors. These people were still steeped in all manner of superstition about sea monsters and mermaids and wandering spirits of lost sailors or whatever, and probably believed as well in the existence of the flat edge of the earth.
I imagine that what you're saying is absolutely true. And yet, I don't think that the beliefs of the uneducated are actually all that indicative of the progress of society.
After all, sure, most people today believe that the world is round. But how many people in the U.S. believe in horoscopes, psychics, channelers, crystals, ghosts, U.F.O.'s, alien anal exams, the Loch Ness monster?
Superstition does not seem to be defeated by the advancement of science.