but even in ancient Greece and Rome there existed some legal protections for slaves

Evidence for this is the comments we have from Athenians on the Spartans who they considered to be exceptionally cruel and bad re the treatment of their slaves. At least that is what I remember from reading Bret Devereaux blog.

Anyway, it feels really weird that ER (wait, I shouldn’t abbreviate eigenrobot to that, that is the name of an anti-semitic youtuber), imagines some moment in time when there was no slavery where it had to be invented (see also the weird modern fetishization of inventions we have), which feels to me like inventing a period before we could lift our left arm upwards. And then also conflating all various forms of slavery with chattel slavery (as you mentioned) is just fucking silly. Reasoning from first principles because nobody in your community is a history expert.