If that's your assumption... where do detranstioners fit into it?
There's this (valid) thing called gender fluidity
Certainly, but applying it to detransitioners is a bit tricky.
Most detransitioners would say that they never should have transitioned, and that they were never really transgender at all. They might use different words, what with the inconsistent vocabularies in use and all that. To make it easier, let's take the case of a female who surgically transitions to appear male, but later detransitions, to the extent possible, to female.
In order to ascribe those decisions, that is the decisions to transition and detransition, to gender fluidity, which is perfectly valid, we would have to say that the female at some point began to feel that he had a masculine gender identity, and wanted to bring his body into line with that gender identity, and so chose to transition to male-ish. (I don't know the exact term for a surgically altered body. It doesn't become a male body, but it does resemble one.) At a later point she realizes that her gender has changed to feminine, and she wants a body that matches her feminine gender, so she transitions to femaie. (In this case her body really is female, but I don't know the full extent that transition reversion is possible, so she is female, but perhaps not with functioning female organs.)
(Note that in the above, gendered pronouns were used based on the gender at the time the decisions were made. So, "he" transitioned from female to male-ish, but "she" transitioned from male-ish to female.)
Having performed the detransition, though, I don't think the detransitioners say that's what happened. An FtM then MtF transition-detransitioner generally says that she was born a woman (i.e. feminine gender) and remained a woman, but had some sort of psycho-sexual issues, and was persuaded that she would be better off with a male-ish body. The transition didn't make her happy and she (remember, her gender was actually always feminine, but she was persuaded to transition) wanted to detransition. So she did.
That's not gender fluidity. If you want to say that this case was a valid gender fluidity you would have to assert that she at one point really had a masculine gender, and is mistaken in her claim that her gender was never masculine.