Non-binary is for sure a 21st century invention - and no, any finite number of small and obscure cultures that acknowledged some semblance of a "third gender" does not negate that fact. It's like goth in that it's people taking perfectly normal human emotions (goths and feeling "sad" or "out of place", NB in feeling some mix of masculine, feminine, or neither) and attaching a special label and subculture to promote a sense of uniqueness, meaning, belonging, identity, etc. I'm almost positive NB will die out in a few decades.

I do believe there is a more material, psychological basis for being trans, at least by comparison. And I wouldn't expect that to go away.