But it's not a contradiction. The mutual exclusion between gender fluidity and altering your own body exists only in other people's heads.
I really do mean this: let's use haircuts as an example. Why is it that even smart people who know that expensive hairstyles are a product of the fashion industry and a money-making exercise in marketing driven by celebrity culture, why is it that even people who know that will get one? Because, first, it makes them feel better about how appealing their appearance is, and second, it produces a noticeable change in the way other people treat them including their bosses and their romantic interests.
And let me remind you - a haircut is very often the very absolute first step in gender affirming therapy! This is not unrelated! I'm not going off on an unrelated tangent to attempt a false equivalence: This is part of it!
So, look, if a person has realized that people like ToneDef are wrong about gender, and that gender is more related to how you feel about your own relationship with masculinity and femininity than about the features on your body...
...then why SHOULDN'T they also decide that if they alter that body, they can get in on some of that self-appreciation for their appearance, and to get in on how society gives a better experience to people who look a certain way? Why does recognizing that something is a purely social convention, mean that you shouldn't still want that social convention working for you? It's only an inconsistency if you still have some kind of acceptance for ToneDef's rulebook.