There is something you are missing, yes.

They do think she is trans. The reason for this is that their target is an ally, who put the gender as "other".

It's not gender policing so much as plain transphobia. Typical brony stuff. They absolutely love being transphobic assholes even more than they love their homophobia (see the dude who, at other time, is arguing that homophobia is justified because homosexuals oppress and attack heterosexuals...apparently...).