You're right that most games don't have the burden of introducing a cis-gender character as such; it wouldn't be a relevant trait to discuss unless it might change over the course of the story or cause conflict. Some games give you complete control of your identity, so there's no reason for the game to introduce a character you just made.In this case, gender identity will cause conflict and you don't have any control over it, so its definitely necessary to tell the player about this character trait.
You're also right that the game already takes measures to tell the player they are playing as a transgender and non-binary person. But they still need to do a bit more, but definitely not a "complete introductory paragraph" :) I think just adding one more line after "You are transgender." saying "You are non-binary." would clear up all confusion. It's not tucked away in the "about" section. The player doesn't have to infer it from the title. Every player will see it.