Isn't this contradicted by the centuries of slavery in our history? Or is the author arguing that the people who were enslaved did not have human-level intelligence (which would be rather a problematic claim)?
This isn't contradicted by millennia (not centuries) of slavery because it was forced on the enslaved populations against their will.
> Or is the author arguing that the people who were enslaved did not have human-level intelligence
He gives an example of "a clever Greek philosopher slave of a less intelligent Roman slaveholder." Does it sound like he's arguing that Greeks were not of "human-level intelligence"? No.
At least, that's what I'm pulling from the quote, have not read the full context.