Subs and communities like that in general only work by genuinely maintaining that confusion. If people see it as ‘only a joke,’ then they can allow themselves to engage in and spread bigoted ideals without ever feeling the need to honestly evaluate their beliefs or feelings. Every criticism can be dismissed by saying “it’s just a joke” and refusing to consider the topic as worth any more reflection.

There are probably some people in there that have no idea the other users are hardcore racists, but that’s exactly what the bigoted users are counting on. It is a strategy to desensitize people to xenophobia and make it feel less threatening.