Go look up eg the sources on the wiki article, and you can see for yourself how they controlled for nurture.
I think the idea that we can control properly is extremely unlikely and IQ will turn out to be a lot difficult to extract from learning/society/environment than you might think, especially as findings with epigenetics and intelligence start to be shown. https://www.europeanscientist.com/en/research/epigenetics-an...
As for cultural loading in intelligence tests, maybe you could expand on that? How do you measure how much culture was involved in the methodology, and how it affected the results of the tests?