Gerald Horne's thesis is essentially correct. The British Empire was more willing to pivot away from slavery and they did so because they realized constant slave rebellions destabilized the wider empire. Moving away from slavery was a worthwhile sacrifice if say, company rule in India was secure. Horne talked about how colonial troops, as in soldiers levied from the colonized population, would be used to put down colonial uprisings from a different colony. This was pretty much practiced by every single major empire like Rome in order to divide and conquer the colonized subjects. There were Algerian troops when the Vietnamese waged their national liberation struggle against the French (and as the story goes, Algerian POWs were reeducated by the Vietnamese and once they returned to French Algeria, they decided to wage their own national liberation struggle). But once an African slave has a musket in his hands and military training, there's no way to re-enslave him without the slave soldier turning on his masters.

Meanwhile, the ruling class of the US white settler population derived their entire fortune from slavery. Obviously, a pivot away from slavery would be completely unacceptable because they would be financially ruined. The idea of armed African colonized soldiers was also completely unacceptable even if used as shock troopers against the Indigenous. Notice how outside of a brief period where buffalo soldiers were a thing, the US never really recruited Black people as front line soldiers in order to steal land from the Indigenous. Black people would be forced to do almost every demeaning and toiling tasks as slaves, but dying for the sake of stealing Indigenous land had to be reserved for white settler soldiers. The settler-colonial state's fear of armed Black people exists to this very day. Why else would Reagan sign that gun control bill into law? It was because the BPP demonstrated with fully loaded rifles.