The entire point of the game is that neither of this cut-and-dry words fit the identity of the person you're playing as. "Nonbinary" isn't one, singular third identity, it's a spectrum of identities and experiences that can't be explained in a way that's easy to consume, or that any cis person would be content with. The point that the entire game is making is that in a cisgender world we're expected to put ourselves in these boxes of male, female, blue, pink, one or the other, something that a cis person understands. The game stating an absence of identity rather than the exact description of the player characters identity is the entire POINT of the game. Because in a binary world, being nonbinary is an absence, it doesn't fit. "You are nonbinary" seeming like a valid fix for this, is a shoe-horned, cisgender way of looking at the point of this game.
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Ahh, I see. I had never considered the game was, "stating an absence of identity." It's an interesting idea that the game is highlighting that absence by clarifying you are transgender but not that you are nonbinary. I could see how telling the player explicitly that they are nonbinary might give the impression that their player character has their identity pinned down, and that would go against the definition you gave for nonbinary.
That being said, maybe there are better ways this idea could be presented. For instance, maybe the game could begin to type "You are nonbi-", but then the screen glitches, and you are presented the first choice. I dunno, something for the developer to decide.
Thanks for giving me such good explanation for nonbinary. It gave me more perspective on this topic.