It's a good quote, and it's actually a little ironic because the line sort of proves itself, as it's actually a myth that people ever believed the world was flat pre-Columbus. Scholars have known the Earth was round for literally thousands of years; Pythagoras wrote about the curvature of the earth as early as ~500 BC. The roundness of the Earth was never really contested until the last 50 years or so.

The myth stems from Washington Irving's book "A History of the Life and Voyages of Christopher Columbus", in which he completely fabricates the idea that Columbus argued with scholars about the shape of the earth (as well as several other stories with zero historical data to back them up). Everybody at the time generally agreed, before Columbus ever set sail, that the Earth is round. That misconception about what the public believed is relatively new, as the book was only published in 1828.

Fact-checking was a much more arduous process back in the 1800s. Back then, you'd typically have to find a book to prove your point, so it's really no surprise that people just accepted these printed words as the truth, but in this case the book is just full of straight-up lies. Lies that eventually made their way into almost every school's history curriculum ever since. In fact, there are more flat-Earthers now than at any point in history, and we can probably directly blame Irving for that, for putting such a stupid idea into the public's eye in the first place.

Interestingly, Columbus was actually WRONG about the shape of the earth. He didn't believe the Earth was round at all. While most scholars accurately believed it to be spherical, Columbus thought the planet was pear-shaped. But "proving" that was never the point of his voyages, either way.

15 minutes ago, some of you reading this "knew" that people believed the world was flat 500 years ago.