both characters are described as being trans in the Japanese versions of the game. when they were localized it got lost in translation and they were only described as using she/her pronouns and nothing else on the matter (aka treating them as if they're cisgender). edit: wanted to add it's prob also partly the ESRB's fault.
it's a whole mess, I just read an article recently abt how they've poorly handled birdo's status as a trans character in recent years, again I can only assume it's bc Nintendo seems to think that being trans isn't a family friendly topic. just my guess though.