Ask the slaves that lol. That argument is moot because it relies on legitimizing the oppression committed by slavers by not seeing enslaved people as part of the population/group. Their history was not recorded the same way the slaver’s history was, yet they were still humans that thought about, talked about, and theorized about morality too. You don’t get to claim to know the group consensus of a past society just because slavers used oppression to erase the viewpoints of those who disagreed.