Because people with XX chromosomes who use supplementary testosterone have a distinct medical profile, whether or not they chose it voluntarily.

It would be weird, in fact, to wonder about trans women, hormones aren't that fungible.

We could also compare the East German athletes with bodybuilders if we wanted, of both sexes and any gender. Turns out they die early as well.

Also, if you're going to go wading into these waters online: the spelling trans men is preferred over transmen.

Edit to add: Trans men includes Buck Angel, so let's not pretend there's an arbitrary limit on testosterone use there. Much like cis men, some trans men want muscles much larger than any normal amount of testosterone will produce.