Given this irrelevance of facts about differences to the moral principle, Singer doesn't really have to address the validity of the findings. But anyway inequalities actually inflicted by society are wildly out of proportion with any of the differences thought to have been found. The phrase is meaningless as an interpretation of fairness, given the actual difference in that exist between different people. The crucial factors are genetic endowment, environment, education, family income etc. All this doesn't mean that it's wrong to hire a qualified employee, if I need someone who is tall, or strong, or able to do physics, or whatever.
Affirmative action seems to flout equality.
Disadvantaged people scoring lower may thereby show higher ability. But the clinching argument is that university admissions aren't based on consideration of the applicants interests, but of the community's overall interests. Nothing precludes a better racial representation from being included in those interests. So admitting more blacks isn't any more intrinsically unfair than admitting people with smart IQ's, or good football players.