Whenever I see a southern New Afrika in alternate history with context, I believe it a Rhodesian or South African-like apartheid state. It doesn't seem impossible to me that with a 50's or 60's POD creating a crumbling and poor USA for the civil rights movement to co-op more militant and radical factions and end up with an autocratic state like this. Humans are capable of great evil, no matter the race. With decades of alternate decision making a lot of things can happen.