So defining transgender people as those who at birth were not assigned the correct place on the gender spectrum has the implication that every single one of us is transgender; there are no cisgender people.

The logical conclusion of all this is: if gender is a spectrum, not a binary, then everyone is trans. Or alternatively, there are no trans people. Either way, this a profoundly unsatisfactory conclusion, and one that serves both to obscure the reality of female oppression, as well as to erase and invalidate the experiences of transsexual people.