No, his arguments were materially different in this case. Most of his arguments came from first principles and worked outwards from some baseline; in particular - what is virtue and how virtue, itself, leads to satisfaction in life, and onward to how this can apply to systems and politics in general. But slavery he treated in an entirely different, practically ad hoc, fashion starting from slavery and then trying to shoe-horn in a justification along the lines of what you alluded to already with e.g. natural order and it being an inescapable inevitability.

It was a complete, and poor, rationalization. He even added, almost as a disclaimer, that there was not a complete overlap between 'natural' slaves and legal slaves, giving himself a plausible out to explain the endless examples of the repulsiveness of the institution by applying a no true scotmans fallacy, 'Ahh yes, I would agree with you there. But that is because that is not a natural slave, but merely a legal one.' And this is not my opinion alone. It has long been considered notably weak, especially from an otherwise brilliant man.

And I think that leads into your next issue. I don't think higher intelligence makes it easier to treat morality as a fiction, but rather even average intelligence, without discipline and virtue, makes it very easy to engage in self delusion and cognitive dissonance. Even those conditions are hardly a guarantee - Aristotle certainly had and strived for both discipline and virtue, yet the desire to rationalize what we want to be true, even if we know it is not, is a never-ending struggle that's easy to fail.