Granted, it's not proof, but I find it very hard to believe that all cases of identical twins with apparently differing gender identities is explained by one of the twins simply electing not to transition while the other does. This is particularly hard to believe given that the twins grow up in similar environments, so if one is in a transition-hostile environment the other likely is as well. I think we should believe people when they insist they are not transgender, especially if they are part of a study where their identical twin is comfortable being open about it. If this were a rare occurrence, I would be more inclined to agree with you, but it is not rare at all.

"transgender"

One instance where I have seen "transgender" used this way is from the same article where I learned about the link between transgender, skin elasticity, and hyperflexibility:

it’s at least possible that EDS and transgender are linked

It's no typo; other articles by this same author use the same grammar. I have also for sure seen this used on other sites, including by trans authors, but in 5 minutes of searching I can't find those instances. "Being transgender" does seem grammatically fitting to me, but it doesn't always make sense to use "being transgender" as a substitute for "transgenderism"*/"transgender." Anyway we more or less agree here and I have little interest in semantics.