There isnt any appeal. It is just them trying to carve into that tiktok market, but the people watching tiktok are already just watching tiktok, so it is basically just there to annoy the rest of us.

Any explanation for non-Americans on what is going on and why this is funny?

It means “brigading”, specifically “brigading by shitpost”, basically “bullying”. They use the term themselves for their own behaviour on communities outside of their own instance here on lemmy, and it is very typical of their authoritarian beliefs.

…by dunking on their user bases

They don’t really bother to hide the fact that their primary reason for existing is trollng and brigading, do they?

Edit: Here is proof that “dunking” is their term for organised brigading, where a hexbear member is posting a comment in another instance on hexbear “if you wanna dunk”.

And the vaccine was a choice. Noone was forced to take it.

It is definitely a hallmark of the politically illiterate to be completely unaware of the paradox of tolerance.