I think with regards to slavery you are bringing in modern views of humanity. I think it was the modern African chattel slavery practiced by the West that introduced the concept that slaves were less than human.
Prior to this, slavery was just considered something unfortunate that happened to a person. A person could become a slave because of debts. A person could become a slave if they lost a battle or their city was conquered. For sure, it sucked, and they did not have any rights, but again, there wasn't really a concept of human rights. In addition, there was no real concept of equality. These were very hierarchical societies. Thus, a slave, I think, would have been considered human, but at the lowest rung of the social hierarchy. In addition, there was no real ethnic component to slavery. There were Greek slaves and Greek masters. Slaves also were not considered intellectually inferior - see for example the Greek physicians and tutors that Romans would have as slaves.
Ironically, it was the push for equality and human rights that made African slavery so toxic. In the past, without concepts of human rights or equality, slavery was just what happened to people who were really unlucky somehow. However, with the concepts of equality and human rights, people realized that slavery was incompatible with that. However, they still wanted to enslave Africans. The way they dealt with this dilemma was to say that Africans were not really human, which magnified the damage caused by slavery a 100 fold, since that horrible idea persisted even after slavery ended in the West.
The GP post is talking about things the happened ~2.5k years ago (greek slavery) and less than 0.5k years ago (african slavery).
It's off-topic. Not incorrect, not unimportant. Just off-topic.
Seems it depends https://en.wikipedia.org/wiki/Slavery_in_ancient_Greece#Slav...
"Xenophon notes the accepted practice of treating slaves as domestic animals, that is to say punishing them for disobedience and rewarding them for good behaviour.[130] For his part, Aristotle prefers to see slaves treated as children and to use not only orders but also recommendations, as the slave is capable of understanding reasons when they are explained"
There's plenty more. I think you're being a bit rosy-tinted here. Some like josephus did ok https://en.wikipedia.org/wiki/Josephus but many slaves got worked to death.
1. The image of the enslaved Greek language tutor gives a misleading picture of ancient slavery. It's true Greeks and Romans seem to have divided their social world on language and religion rather than skin color. But the massive numbers of Carthaginians and Gauls worked to death in copper mines were almost certainly not seen as equals by their enslavers.
2. I may be misreading you here, but the claim that "it was the push for equality and human rights that made African slavery so toxic" is reversing cause and effect. Racial plantation slavery in the Canary Islands predated modern movements for universal human rights. It was slavery that made human rights a European-only idea, not human rights movements that made colonial slavery a racial institution.