Being a flat-earther is a rhetorical exercise. It's a game designed to force you to give extremely convoluted but clever responses to extremely simple common-sense arguments. I used to force novice college debaters to play "Flat Earth," "Cheese Moon," and "The Sky is Pink" early on to demonstrate some subtly deceptive argument forms. You'd be amazed how good people get at this game. Stephen Colbert essentially plays a variation on this game all the time when in character.
There are probably a couple loonies who really believe in a flat earth, but I bet most people on that discussion forum are just playing "Flat Earth" and letting their troll targets come to them.