To quote some actual research on CAIS:
Complete AIS (CAIS) has a frequency of approximately 1:20,000 male births. CAIS is characterized by 46,XY karyotype, female external genitalia, testes located in the abdomen, inguinal ring or labioscrotal region, and internal genitalia with a blind ending vagina, absence of uterus and fallopian tubes, and no Wolffian development [1].
https://pmc.ncbi.nlm.nih.gov/articles/PMC8026333/
It's a very rare disorder of male development, which leaves the person's outside appearance with some female phenotype markers and internally with some male ones. Socially, most people with CAIS (but not all) express a gender that is consistent with being female. That doesn't actually make them biologically female. And it's not the gotcha you think it is.
As someone who has experienced numerous investigations as a child for disordered sexual development (not CAIS), it's deeply, deeply offensive to be co-opted into the gender identity arguments.
My biology and therefore sex (incontrovertibly female) isn't a choice. The way I present my exterior and behave (what I think many would call a "gender non confirming" way) is a choice. My sex doesn't change based on my behavioural choices. My behavioural choices are influenced by social conditioning, rejection or acceptance of societal gender stereotypes, etc etc. My sex is just the way I was born.