Trying to ascribe the intent of an action rather than focusing on the action itself is part of the problem in this circumstance. It's impossible to know how many are sincere, or how many are trolls pretending to be sincere, or how many are sincere but are pretending to be trolls so as to plausibly deny their sincere beliefs, etc. At the end of the day, the intent doesn't matter. If one is in a public forum (especially an anonymous/pseudonymous public forum) then the only thing to judge is one's actions, not their intentions, and if their actions are deliberately stupid, then we must assume they are simply stupid, and not merely pretending to be stupid. This approach obviously has problems, but there's no other tractable solution. The internet (especially Poe's Law) has killed satire.
"Irony", or what passes for it these days, is too often a deliberate tactic to equivocate a sincere belief while retaining plausible deniability. Unless they deliberately call out their comments with something like an explicit sarcasm tag, we are forced to assume they are sincere.
This may simply be a function of your inability to believe that they could be sincere.
There are without question plenty of "real" flat earthers our there. People willing to spend actual money on experiments doomed to fail. People cutting ties with their families because they don't agree with them.
When we read people espousing flat earth views on YouTube or Reddit or wherever, out natural assumption is to think that they must be trolling, because nobody could be that stupid, but in fact there really is a subculture that does.
