West Africa was cursed by their geography
There are very few navigable rivers in the region, and massive rainforests separate hundreds of different African tribes.
There's a reason why countries like Nigeria and Congo have 300 different languages. These people developed independently of one another, because of the shitty geography. And the Africans didn't have many horses that could cross this hostile terrain, making them even more isolated. The Sahara desert made it nearly impossible to reach out to Asia or Europe. Very few men were capable of crossing that hostile desert, and only in small numbers at a time. No way could massive armies cross that terrain on a regular basis.
However, they did manage to create empires in modern day Mali, Ghana and Benin. So this pastor isn't telling the truth. There's a depth of knowledge detailing the many African states, kingdoms and innovations at that time.
East Africa was different though, because the geography there is much more suitable. Hence why the Somalis were able to reach out to different civilizations.