The point isn't that the non-flat-earthers are actually wrong, it's that we don't know whether they're wrong or not.
How does that follow? If we have a thread discussing Jainism it's only reasonable if there are Jains in the thread? In the modern world it's possible to learn about topics without actually being there in person. I have read about WWII even though I wasn't involved, but I still trust the facts as they were relayed.
Further I'm actually able to confirm some of the things said in this thread by watching videos or reading material put out by flat earthers.
So yeah, we do know if we are wrong or not.