Are you referring to their increasing average IQ? If so, yes this is simple to answer.

Living standard are increasing rapidly all over African countries. Meaning the new generation of people have adequate nutrition, exercise, and education among other factors. All of this plays a role in increasing their mental capacity. So their IQ is increasing as consequence.

This doesn’t negate what I said earlier about the upper limit of IQ, however. It is still capped by genetics. The people in Africa were not able to tap into their IQ potential because of previously poor living standards.

A good comparison would be athletics. Everyone has a peak athleticism stage, which is determined by genetics. But if you starve a kid, obviously they won’t ever such a stage.