I dunno about the specific science you're referring to but I've typically heard "trans medicalism" used to denigrate people who think that you can only be trans if there's a medical explanation. So if you believe that someone has to experience gender dysphoria to be trans, then that's trans medicalism and that's bad because the truth is there's a lot of different trans experiences out there that don't all conform to the dysphoria narrative. AFAIK it's somewhat common for trans people seeking medical care to simply say they have dysphoria and use certain "key words" when describing their experience, even if those words are not entirely accurate, because the goal is to get care and most doctors are looking for those key words and they might not be fully up to speed on how diverse and sometimes contradictory peoples' feelings can be in practice.

There's also a logic trap you can get hung up on with the medical arguments that support the existence of trans people, which is that you might accidentally imply that it's only okay to be trans because there's a medical component. Always remember that even if we never find any "explanation" for why trans people exist, that doesn't mean that they don't - and on the flip side, even if we proved conclusively that being trans was optional and that people were "choosing" to do it, that still wouldn't make it any less valid of an identity to have because we shouldn't believe that there's anything wrong with it.

But yeah as long as you're not saying that it has to be medical then I don't think you should catch side eye researching the medical side of it, since there definitely is one. Although as some here have pointed out brain scan studies have very limited value.