The irreversability is only diagnosed after one reaches puberty so it seems that one cannot be born a transsexual but one can be born with GID.
How you draw that conclusion from the research is hard to follow. How does the time at which the irreversability has been diagnosed determine when the condition started to exist. Your conclusion is not supported at all. The correct conclusion would be that transsexualism as a condition only occurs in 2.5 - 20 % of those cases reviewed by this paper. Since you cannot diagnose irreversibility until onset of puberty the existence of the condition cannot be conclusively diagnosed until that time. It has nothing to do with whether you were born with it or not.