From the article:
He has also noted that the IQs of the children's mothers and fathers are unknown, and that white and black G.I.'s in Germany were not equally representative of their respective populations, since about 30 percent of blacks, compared to about 3 percent of whites, failed the preinduction mental test and were not admitted into the armed forces. He further argues that the selective preferences of the German women with regard to sexual partners may have influenced the results in an unknown manner.
Yeah, you could play all of this out for quite some distances trying to separate out culture, genetics, environment. It’s probably the case that even if you somehow equalized everything except pure biology (of course impossible to do), you’d end up with some differences. Certainly that’s the case for other less controversial topics such as height, athletic ability, etc. Having grown up in a mixed-race family and now having supervised dozens of employees of every race for the past decade or so, I suspect that differential would pale in comparison to the cultural and environmental factors that are currently driving things for the most part.