I don't think the case for Moldbug-as-pro-slavery based on that article rests on picking two sentences from it. Here are a few more.

Fairly early on, he argues for a more modest proposition:

"A society is richer if each individual in it has the right to bind her future actions by agreed obligations, in return for which others may exchange other consideration. [...] we are almost all the way to Carlyle on slavery. We have not agreed that a man can be born a slave, but we agree that he can sell himself into slavery. That is: he can sign a contract with a master in which the slave agrees unconditionally to obey and work for the master, and the master agrees unconditionally to protect and support the slave."

OK, but that's only voluntary slavery. What next?

"It is only a short step from seeing the State as an enforcer of voluntary and binding obligations, to an enforcer of involuntary and arbitrary obligations. No society can possibly exist without uncontracted obligations."

He goes on to give (not being allowed to trespass on) inherited property and (the obligations that come with) family relationships as examples of uncontracted obligations.

OK, so Moldbug is in favour of voluntary slavery and thinks that sometimes it's good for the state to enforce involuntarily acquired obligations. We're still not all the way to actual slavery, of course; it depends on what sort of involuntarily acquired obligations the state should be enforcing.

"Indeed, the definition of family in most times and places has included slaves. [...] The relationship of master and slave is a natural human relationship: that of patron and client."

So now he tells us that slavery is natural and suggests (though he doesn't exactly say it) we should think of it as an aspect of family. He goes on to discuss slavery (being bound to one person), serfdom (being bound to a patch of land and its owners), and ordinary government (being bound to a much larger patch of land and its owners -- it seems to me like there are some super-important differences between serfdom and citizenship, but never mind), and concludes from how widespread these systems are:

"This is a pretty good clue that this structure is one to which humans are biologically adapted."

OK, so he says slavery is natural and humans are biologically adapted to it. Now what?

"Not all humans are born the same, of course, and the innate character and intelligence of some is more suited to mastery than slavery. For others, it is more suited to slavery."

(It is interesting to put this alongside his protestations that when he says black people are less intelligent than white people, of course he isn't at all making any sort of value judgement or saying that white people are better, dear me no.)

OK. So he says slavery is natural, humans are biologically adapted to it, and some people's inborn characteristics make them natural slaves. What next?

"These characteristics can be expected to group differently in human populations of different origins. [...] Africans tended to make good slaves and Indians did not. This broad pattern of observation is most parsimoniously explained by genetic differences."

So now we see the passage you quoted in its context. He says: Slavery is natural, humans are biologically adapted to it, some people's inborn characteristics make them natural slaves, and these characteristics are distributed differently in different populations; for instance, at least a few hundred years ago, Africans made good slaves, and this is probably for genetic reasons.

(But oh deary deary me no, when he says there are statistical differences in certain psychological characteristics between races, he doesn't mean any sort of value judgement by it. Only that some races are naturally suited to slavery and others to mastery, that's all. Who could possibly object to that?)

"Conversely, the subject-government relationship easily becomes dysfunctional for clients who are natural slaves, ie, are not capable of guiding themselves to live in a human and humane manner. [...] If all long-term welfare cases were transferred from Washington to the authority of genuine, truly charitable nonprofits, for example, their new human supervisors could intervene on a personal, discretionary basis to compel them to get their acts together. This would be a step toward humanity in our society - and also a step toward slavery."

So: he says slavery is natural, some people are natural slaves, those people won't flourish as free citizens; "all long-term welfare cases" are in this category; they should become, in effect, slaves -- for their own good, of course! -- and this would be an improvement.

(But no, of course he isn't an outspoken advocate of slavery. How could anyone think such a thing?)