Slavery is mentioned in the Bible WAY before 800 AD.
Yes, and I believe that the reason that it was not widely condemned is that it was used as an effective way to treat subjugated nations...to keep them from rising again.
The author specifically refers to the
modern type of slave trade. Previously slaves were not bought and sold in batches except after wars when a good percentage of the losers became slaves of the winners. Without the possiblity of profit from selling or using those slaves they would not have been captured. They would have been killed.
Many more people "sold" themselves into slavery to avoid starvation. Judgements in lawsuits were another source of enslavement as losers tended to not have sufficient property to pay a judgment with money or goods.
Slavery was also an alternative to execution for criminals. Societies were not rich enough to afford years of food& for convicts so misdemeanor theft got you hung, or impaled, or something. No, the Arabs modernized the slave trade 1200 years ago. Before that enslavement was mostly an alternative to immediate death.