Yes, he picked the wrong venue on the wrong side of Africa. Around 17 million Africans were sent to the Arab/Muslim world as slaves over 14 centuries, with Zanzibar being one of the more active departure points. The European/American involvement in slavery lasted about three or four centuries and the Europeans were largely responsible for ending most of the African slave trade.
Obama could have covered many more centuries of the slave trade which involved many more slaves in east Africa where some slavery is still practiced.
Please tell us some details, and then we can rally against those evil WHITE plantation owners who are still operating in east Africa!
...oh, wait, what? There aren't...but how could you have slavery without evil white people?
...OK, forget it, I'll just go back to putting the finishing touches on my Justice for Trayvon memorial hoodie. Forward!