Looked the silly notion up.
Seems that a cis person is one whose “individual experience of their own gender matches the sex they were assigned at birth.”
Note the weasel wording. A baby isn’t born a girl. She has gender identity as a girl assigned to her, apparently randomly and certainly not legitimately.
Good observation. When it says “they were assigned” their sex (are we all committees now?) at birth, it’s saying that sex isn’t a biological reality. Of course, one has to say that if one is going to claim that sex can change.