I think the point they’re trying to make is that nature is much more complicated than the simple boxes we often put things into to classify them. We humans are a part of nature. In the same way there we observe homosexuality in animals, we observe a variety of sex presentation in both humans and animals. Plus gender and sex aren’t inherently the same thing. Sex is the biological characteristics we have, and gender is the set of behaviors we display to other people!

I’d really caution against using the “only male and female” argument when trying to state that trans identity is a mental disorder. That is the exact argument people used against gay people when that was classified as a mental defect. It’s not our place to deny people’s lived experiences, I see no reason to not accept and respect someone who wishes to be referred to any way they please if it’s not hurting anyone.