[2020] Acts like a boy, looks like a boy, dresses in male clothes, has a male name, yet somehow he's non-binary? Is there any reason for this other than having Kotaku write an nice article about it?

Yes.

The reason is that we decided for Max to be non-binary because we create the character and we give them the personality we want them to have. The same way Aina is an old lady, Konstantinos a middle-age man, Carlos a young lad or Tihani a young female.

You don't need to find obscure reasons for everything. Temtem has over 700 NPCs and each one of them is unique. Same that happens here on the Earth.