Also, he was referring to people performing labour, not people adhering to spiritual belief or ideological systems.
I see what you mean, but I don't think this means what he believed was true. I see it as categorically different. People adhering to beliefs without critical thought vs being forced into labour due to their 'nature' are very different matters.
many people are meant to be slaves is also a fact -- it is not like everyone is gifted cerebrally, or even blessed to be in an environment where, by the time they are adults, have been shown a good formula for life; so consequently a section of the society is destined to be just slaves, it just needs the presence of a manipulator.
Aristotle, being an endless questioning guy he was, might have figured this out, although i'm not sure.
I think you're committing a naturalistic fallacy, and I don't agree at all that people are ever meant to be slaves.
The most I can agree to is that "some humans are born into slavery due to chance". This is factual. That people are meant to be slaves is not a fact. Perhaps we have a language barrier here.
I've no disagreements with your position or moral principles. But spherical cow is different from reality.