Ah gosh, I missed that, sorry. I know what you mean though. Have you read "Gender Trouble" by Judith Butler by chance? It kind of explores this question. Basically, its thesis is that gender is not something inherent within us, but rather primarily something we do/perform instead.
But at the same time, even if transgenderism doesn't exist prior to gender (but rather a result of gender, another structure), I don't think that would negate its legitimacy. And most other queer/feminist theorists (excluding TERFs) would likely agree. Because gender, as it stands right now, is perhaps foremost an act of signification between an individual and society, I believe how that individual expresses themselves should be dictated by them, rather than subject to societal hegemony. If we think of the whole performance of gender like a play, where males and females have different roles, just because they're ultimately roles and imaginary doesn't mean that one of them isn't ultimately much more "our role to play." If the whole binary breaks down eventually (which is what I personally hope for), obviously there won't be a need for the concept--as these innate qualities in people won't need to be signified through the concept of gender. But as studies have shown, people identify as trans for so many different combinations of reasons, and there will inevitably be individuals who still feel they would be more comfortable with breasts or without a penis for example, even once the binary disintegrates. It's just that this act would no longer be considered "trans," but rather regarded simply as a natural part of normal life and biology. I haven't thought through this enough to clearly articulate my thoughts, so I'm sorry. But I hope that kinda makes sense