One of the reasons I'm skeptical of him is that he's extremely elusive for such a major (and divisive) figure. For info on the origin of white supremacy and the evils of settler colonialism I steer people towards Gerald Horne, who is a respected black marxist historian.
Gerald Horne is incredibly good, J Sakai has some issues with citations or sharing methodologies as well, imo. However I've found Settlers to be a good "wrecking ball" for dismantling perceptions on history so I tend to recommend people start there and then move on to The Counter-Revolution of 1776 or something similar
Horne's good. I do think the counter revolution of 1776 over plays its hand a little bit though. To me the revolution is more about settler colonial revolt over conceptions of freedom Ala Aziz Rana's the two faces of American freedom, of which a key part is anxiety over slavery law, but not the principle component (and there is no principle component it's a mixture of several).