performance on IQ tests is mostly related to culture

Shalizi simply doesn't say that.

There are two things you could mean by it. One is that some cultures make you smart. The other is that the IQ test mostly screens for culture and not useful abilities. It is certainly true that culture affects the difference between performance on Raven's matrices and other tests. In particular, the Flynn effect is stronger for Raven's matrices than other tests. Also, sub-saharan Africans do dramatically worse on RM than on other estimates, where they're closer to African-Americans (who do slightly worse on RM than on common tests). In applying this information to the two possibilities about culture, you'd have to decide which testing approach you liked better, which would depend on what you're trying to measure. "g" is not the correct answer to this question.