He's saying the enslaver wants contradictory traits in the slave, intelligence and subservience.

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.

The very same slavery which resulted in the civil war and literal killing of hundreds of thousands of non slaves, followed by their freedom? Or the prior enslavements that very often ended in organized rebellion? Slavery is at most a temporary phase when it involves beings of equivalent intelligence.

Is that complete subservience ? Slave history has tended towards slaves no longer being slaves over long enough time horizons, and not simply because the slave masters were just feeling extra nice. Slaves don't really like being slaves.

"Rebellions of slaves have occurred in nearly all societies that practice slavery or have practiced slavery in the past."

https://en.wikipedia.org/wiki/Slave_rebellion

Neither, the author is pointing out the desire of the enslaver, not the actual outcome. But I don't think their logic takes into account access to means to 'outsmart' the enslaver. It's trying to frame it as a single instance equation, not a societal one to try and show the underlying contradiction of desire.

At least, that's what I'm pulling from the quote, have not read the full context.