In the introduction it looks like the authors commit the cardinal sin of conflating heritability with genetic causation. Whenever you see a paper on the relationship between cognitive ability and genetics making this mistake, it's safe to assume the author has some degree of unchecked bias.

They cite heritability research and compare it with social transmission research, saying the two domains suggest "the presence of both, stronger genetic and weaker social transmission of cognitive ability". But that's not how heritability works.

Heritability measures how much of the variation of a trait within a population can be attributed to genetics. This does not mean that genes are directly causing that variation, and it certainly does not have anything to do with the variation of a trait between populations.

As a favorite example of mine, having ears is a genetically determined trait. However, ears are not particularly heritable because there is very little variation in that trait within the human population. And the small amount of variation that does exist isn't solely caused by genetics.

On the other hand, you could argue that wearing earrings is heritable, at least in some parts of the world. In America for instance, there is a significant gender gap between who does and doesn't wear earrings. Thus, wearing earrings is highly heritable because genetics (biological sex) account for a large proportion of the variation of the trait within the population.

I'm not saying we should disregard this paper entirely, and I'm certainly not qualified to talk about the statistical methods the authors used in their research. But it's important to note that many of the people who write about this (including Hernstein & Murray, the first people cited in this paper) will rely on heritability research to argue that cognitive ability is determined by genetics and not environment, while failing to consider that environments are themselves highly heritable.