The site is from the 14th century. Europeans came to central Africa in the 15th century when there were still many well organized kingdoms and societies there. European interference in terms of guns and the slave trade was a major part of these cultures dissolving.
The GP is presumably referring to the Ghana and Mali Empires, which did arise in large part through ironworking, and then declined due to internal strife some time before European colonialism took off in the region (the former empire collapsed centuries before the Europeans arrived).
I was thinking more of the kingdoms of Kongo[0] and Benin[1] that are geographically closer to present day Gabon.