They/them is not a real thing. Non-binary is an identity, which means non-binary people have actual genders. They just don't accept them.
This goes for all identities, really. There are only 2 genders, but some people want to be special, so they invented their own.
This is why we've had to move on to calling it "sex", rather than "gender". Because gender has now become an identity you can create at your own discretion, while sex is their actual scientific gender, outside of the fantasy realm.
Please don't correct people with made-up science from a minority of insane people. You might as well correct someone and tell them that the earth is actually flat. Sure, there are people who believe it. But that's what separates us from the... "dumb-dumbs". (Trying to be nice here)
Similar to what you said. I'm not aiming to be harsh. Or put you on the spot. I'm sure you've been fed those lies on a daily basis, so you don't know any better.
Either way. Since you agree that the main character is non-binary, then the character can be of either sex.
If you think she looks like a girl, then she's a girl.
If you think he looks like a boy, then he's a boy.
I'm sure my comment will be downvoted to oblivion if not just outright removed. Because, well, the truth is unacceptable these days. It's all about inclusivity and nice words. The imaginary world takes precedence over reality.