Late to this party/honeypot, but here's my 2¢. I am not a transmedicalist and never have been. That said, I think I may have some insight as to why people are.

In speaking with other trans folks, I get the sense that some folks care immensely about gender presentation and how they are received by others while not caring so much about changing their bodies. Other folks are less concerned with gender presentation, but are deeply concerned with their body being "wrong." Personally, I care about both, but my dysphoria almost always stems from my body rather than from things related to presentation or even other folks' reactions. I have always had a deep sense that my body was wrong and have dreamed of "fixing" it. FFS and HRT have been godsends to me and I am super looking forward to getting bottom surgery as that's kind of the big remaining pain point for me and bringing my body closer into alignment with my internal experience is really important to me.

In short, my experience and personal needs could be pretty-well addressed by transmedicalism (or at least some imaginary version of it that is informed-consent based). If I were narcissistic enough to hold my own experiences and feelings up as a metric by which to measure other trans folks and lacked any empathy, I can see how I might espouse some version of transmedicalism.

Anyway, that's my best guess- that transmedicalists come from a place of being pretty body-focused internally and then lack the insight or humility to understand that other people experience gender/sex/transness in ways that are different, but still fucking valid.

We all believe stupid things at some point in our lives. As @ada@piefed.blahaj.zone implied, sometimes the thing that is needed isn't to root out wrongthink, but to have folks in community where they can see a different point of view and hopefully grow, or at least come to understand others' point of view.