I think the argument he makes consists of several claims:

1) black people are an ethnic group or race // exact terminology and semantics is debatable

2) being black means you have a lower IQ (on average) // true in the US

3) IQ is in part genetically determined // true

4) the lower IQ of blacks is due to genetics // pure speculation, not possible to verify due to poor environment

5) having a lower IQ makes you a better slave // speculation

6) being black makes you a genetically better slave // follows from 4 and 5

It's possible that Moldbug reconsidered claim 4 due to all the attention it got. He reads HN.