IQ is not "mostly random". It's highly correlated with the quality of your education. Kind of like how height is highly correlated with childhood nutrition, and sporting ability is highly correlated with practicing sports.

The article is about how hard it is to tease these confounding factors apart. A highly intelligent parent is likely to prioritize their children's education. A hypothetical study that concludes "intelligent parents raise intelligent children" could easily be construed in favor of either the heriditarian or blank-slatist perspective.