...I wish I knew. It's really hard to tell what other people believe at a fundamental level, and there's certainly a humorous undertone to nearly all flat-earth evangelism I've encountered. But what's telling is that I've pretended to be a flat earther myself when mocking climate sceptics. In terms of the basic trust in many other people OR basic competence in the realm of physics required not to hold a belief, climate change scepticism and round-earth scepticism actually seem fairly close to me. And I'm sure there are sincere climate sceptics.

Flat-earthism is a noncontroversial and extreme example of a belief that gets less believed when its proponents have the full benefit of free speech, but I think there are many more like it.