To be more clear, HBDers claim that not just that humans differ significantly at a genetic level (that's pretty uncontroversial: I don't think anyone is going to argue that genetically inherited disease aren't a thing for example). As far as I can tell, the HBDers believe that most or almost all of mental traits are genetically determined. Moreover, HBDers seem to generally believe that these genetic traits are distributed in the population in ways that closely match with what are normally seen as ethnic and racial groups, and that that explains most of racial differences in IQ scores, life success, and rates of criminal activity.