it doesn't seem to me that he's saying trans people aren't a legitimate thing. My read is that he's saying that gender is related to sex in the sense that your

it doesn't seem to me that he's saying trans people aren't a legitimate thing. My read is that he's saying that gender is related to sex in the sense that your gender is somewhere in a spectrum of male-female (in a similar sense to masculinity-femininity). Which every single trans person I've ever met or spoken to fits into.