Obviously AFAB(1) is totally meaningless factually given that sex is set at conception and in well over 99% of cases accurately observed at birth.
However, words are just sounds outside the meanings we give them, so if this particular sound, regardless of its origin, will allow us to maintain female-only rights, spaces, protections and opportunities, maintain a female political presence and voice, and speak of the common experiences of being female without constant demand that we include males who (offensively(2)) believe they have a mental kinship with female bodied people, I'm all for it.
Cue apropriation of AFAB by male-bodied people, incoming in 5...4...3...2...1....
(1) We can always backronym it to Actually Female At Birth
(2) Offensive because it is not possible to hold this belief unless you also believe that (a) there is a mental sameness between women, and (b), you, despite observing them only from the outside, have nevertheless accurately perceived it.