RE: Sinners are Slaves until they become washed in the Blood of Jesus Christ, by accepting the Sacrifice made on the Cross.
This article is not talking about slavery to our sinful nature. It’s talking about human beings enslaving other human beings. Let’s stick to that particular definition of slavery.
Old testament masters were required by religious law to treat their slaves as human beings. Slavery was a ubiquitous condition. There were no societies above bare subsistence hunter-gatherers who did not have slavery. With periodic famines (one reason) people sold themselves into slavery to survive. Many slaves were human booty from people defeated in wars which would seem to be generally more humane than simply beheading everybody. Slavery was actually a natural condition of the level of civilization. Christians were enjoined to treat their slaves well and slaves could be Christians with all the rights and duties
in the Church as anyone else. God did not say to free the slaves. God was and is concerned with personal redemption no matter what your status in no matter what kind of society. The political system is of no matter beside the personal relationship to God of the individual.
Within the workings of the political system individuals are answerable to God for their own sins. Kings and tyrants are personally liable for their sins. If you seek justification for your preferred political system or condemnation to one you dislike in the Bible you will find no help. If you look for your own personal redemption then it is all there. What matters is the afterlife, not this life.