At first.... many of these African tribes simply converted... There was literally No need for forced conversions at the time because entire Kingdoms were converting. Trade was much more important and Islam brought certain rules and unity to certain detached tribes.
Some of the groups that converted were already trading with the world and had technology n knowledge. So the 'Arab's (as a modern concept) race didn't really sweep through Africa.... Much of it was these Muslims that swept through.... not as a race, but as a group of black and brown people.
And even in places like the Sudan... we have it all confused because we only know what the US school system tells us. Those people still practice ancient african religions.... even if they are Christian or Muslim... they hold on to their tribal shyt. Islam just united madddd tribes that really had no reason to be united before.... so once there was an "african" Muslim group --- That group went on to take over North Africa all the way up to Spain. before that one AFrican tribe felt ZERO connection to the next african tribe.. Not like we do now as 'black' people.
When we talk about the black people who did all that shyt... technically we are talking on black arab muslims. Arab isn't a race. Some are Asian, some are African.