Remember that AHF is the thesis that

Necessarily, something is a woman if and only if it is an adult human female.

Let me unpack that “if and only if.” The above statement is really a conjunction of two claims, namely:

1. Necessarily, if something is a woman then it is an adult human female.

and,

2. Necessarily, if something is an adult human female then it is a woman.

Byrne’s third argument is that it seems that we know that someone is a woman just by knowing they are female, and that we know that someone is female just be knowing they are a woman. The simplest explanation of this is that AHF is true and that we know that AHF is true.

Far away for culture war and seminar room contexts, yeah, it seems that we all do know AHF. As Byrne observes, bringing in a scientific context,

Mitochondrial Eve . . . is the most recent common matrilineal ancestor of all humans alive today. We know nothing about her life or opinions, except that she is human and had children. Nonetheless, as the New York Times (correctly) puts it, she is “a woman from whom all living humans inherited their mitochondrial DNA.”

The reporter here is relying on his or her knowledge of 2 above. Byrne adds that

A similar explanation is available for our knowledge that “Y-chromosomal Adam” (the most recent common patrilineal ancestor of all human males alive today) is not a woman . . .

namely, that we all know 1 above. (By 1, if someone is not a female then they are not a woman.)

I would add: how do we know AHF? Simply by grasping the concept woman, we can (mentally) “see” that it is no more or less than the conjunction of these concepts: adult, human, and female.