Steer away from countless confounding factors and come to the essential parts:
The repressive restrictions of slavery lead to poverty – deprivations of income, capability and optimism, which in turn lead to diseases, physically and mentally, comorbidity and malnutrition – these are typical sufferings that people live in liberty can, in comparison, largely avoid. There are plenty of scientific studies about the links of disease and poverty.
So in the light of health, slavery is the ideal incubator of diseases, especially infections like the pandemic. To make this planet a better place, get rid of slavery.
That is why liberty is better than slavery.
Sam Harris talks about the objectivity of morality, rather than absolute morality. Your misunderstanding has led you to your confusion.
One of the fundamental principles in science is the Uncertainty Principle, which prevents 100% accuracy and thus rejects the possibility of absolute morality.
Different societies have different forms of marriage. What problems are there when you realize the well-being of the people is top priority and the non-existence of absolute morality?