If you notice there were 4 identified Forest States. These were the empires that Esau traded with during the slave trade in that region. The Kingdom of Kongo is not on this map, but they were another empire that did slave trading from Congo/Angola.
Notice that these states were established around the exact same time that Esau began to do his conquering of the Americas. If you look up the Asante Empire, Dahomey Empire, Oyo Empire and Benin Empire. They all had strong partnerships with Esau. Those are the people in Africa that got rich off of the slave trade.
The Igbos came from the Nri Kingdom which began to decline because of attacks from to separate groups Igala and Benin which led to them being major victims of the slave trade, primarily being sold by the Benin Empire through what is identified in the slave trade as the Bight of Biafra. Again, this is well documented. That is why Igbos are the most heavily documented victims of the slave trade although they were not the only groups that were purposely targeted and victimized.