We're much more "Marine" than the Grey Knights or Deathwatch, who are actually ADEPTUS ASTARTES. [Also, until about september, we were appreciably better. Even post supplements, we're still one of the best counter-marines there is thanks to the Order of the Valorous Heart. Don't need no genetic augments here, just blind zealotry!]
And as far as the other explanation go, I don't really find it objectionable. The requirement that the individual be AMAB because they're working with the stunted Y-chromosome is pretty reasonable to me. Requiring the individuals identify as male is well, sexist and kind of offensive.