"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.

In my culture irony equivocates amusement, not sincere belief.

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.

You want to say irony must be imagined and can't be seen?