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.