you cannot compare males and females have the same everything.. we have alot of difference. Physique. Sex cells. Genitals. Just because the emperor was a female at one point doesnt mean he has the genes zygotes of a female.. by saying this you are in my view calling gender a social construct and not biological.
A person doesn’t have zygotes. They were zygotes at one point. You’re acting like females and males are two different things altogether. We have the same DNA. Exactly the same. The major physical differences in gender appear after puberty, which is too late to become Astartes. Testosterone is never mentioned in relation to Astartes. You’re just being willfully ignorant at this point.