The study controlled for cognitive abilities, it's graphed on figure 8c. "Consistent with previous studies linking AFQT scores to racial differences in adult outcomes (for example, Neal and Johnson, 1996; Cameron and Heckman, 2001), I do not interpret these scores as measuring innate endowments but rather as reflecting the accumulated differences in family background and other influences that are manifested in test scores." Please don't fall for naive evolutionary justifications for discrimination and don't criticize blindly.