Slop.

They just assume that because they don't know any trans people, there's no way so many folks are trans in one place online.

If anything, this website is more inclined on hunting down and prosecuting the cis. They'll never catch me though

I think it's just their internal rationalization for why it's okay to misgender people they disagree with, and then if you ask them to stop that's just more evidence for them that you're trolling.

Even more than that, let's actually assume for the sake of argument that in some sense, this was the case for a nonzero number of posters:

• Maybe in 20 years time, there's a new word to describe some other very real type of dysphoria, and it turned out that, say, a hypothetical trans woman on here was actually a cis woman with zero gender feelings at all (there is somehow zero overlap between this hypothetical dysphoria and gender dysphoria) and only this dysphoria. She just really needed another person to talk to about her experience who could relate at all, but she felt compelled to restate things in terms of gender instead.
• Maybe it turns out that, say, a hypothetical cis man on here was only dissatisfied with the cultural baggage tied to masculinity, but he ultimately needed the space to explore their gender identity and realize they felt like being a man best described him after all.
• Maybe it's something else I can't think of right now.

Does any of these scenarios actually justify treating those people any differently than someone who definitely is transgender? I don't see why any reasonable person should come to that conclusion.