This valid point reminds me of the sketch of the cave man with an arrow through his head saying, “Ha, you missed the vital spots!”. So it is with Slate.
But on your point, the African Muslims were prohibited from enslaving Muslims. So the people they sold for slave exportation were animists. That had to have delayed the arrival of Islamist to the Americas and Western Europe in any noticeable numbers for several centuries. Thus, BHO’s progenitors stayed in Africa after they sold his wife’s ancestors to GHW/GW’s. Sort of interesting how all the treads of history work out.
Of course, there is a loophole. If they are apostates (as Shiite consider Sunni, Sunni consider Shiite and Alawite and Wahabbi consider every other Muslim sect), then they really aren't Muslim and said prohibition doesn't apply.
But you make an interesting point. More than likely BO's forefathers never saw a need to emmigrate from Kenya because they were at or near the top of the heap.