I agree with Judith. Assigned at birth terminology is attempting to convey the same sort of thing that Judith is in the quote. Gender and sex don't arise from the body, they are applied or assigned to it by various forms of power. They aren't based on a holistic understanding of what the body is or who the person that inhabits the body will become. They are based on the perception of a few small external features of the infant. And when those are inconclusive, doctors perform surgery on the infant to assign a binary form based on how they think the body should be gendered.

I will note that these surgeries on intersex babies are not what fascists and transphobes refer to when they describe gender affirming care as mutilating children. They are perfectly ok with surgically altering a person when it enforces a gender binary. They also don't advocate for the banning of circumcision, another form of cultural genital mutilation.

Even sexual dimorphism isn't a binary, but rather a distribution with a lot of overlap. In most medical situations, amab or afab carries a lot of assumptions that may not be true but nevertheless the treatment of the patient will be based on those assumptions and may harm the patient. For example, HRT changes the risks of certain illnesses and a doctor ignorant to that (a common problem for trans people getting healthcare) wouldn't treat the patient appropriately. This is another example of sex being gender applied to the body through a material process. It's not figurative. It has real consequences that are played out in physical bodies.