It is a wonderful prank.
For a long time, skeptics and innovators have been chastised with the (usually irrelevant) observations that :and people once believed the Earth was flat”. Political opponents on a scientific question were also derided as belonging to the “Flat Earth Society”.
Arguing for the absurd with one’s most hated arguments can be cathartic and relieve the stress one builds up after listening to many logical flaws in scientific and political debate.
Actually, it's a myth that we've only recently discovered the earth is round. According to Wikipedia, the round earth has been the prevailing theory for centuries ... though, for ordinary plebs living in the pre-telecommunication era and in one place all their life and never doing any sailing nor significant travelling I suppose the flat earth point of view was easier to accept than the round earth point of view.