ultimate threat (if you don't sell your neighbors, you can't get weapons to protect yourself) came from Europeans and Americans.

The section on causation (5.1 in particular) seems to suggest the other direction. The areas that were not up to trading with Europeans- the ones that didn't have national currencies, economic institutions, lower population density, etc.- were less touched by slavery, because they'd just attack Portuguese traders who sailed up (because raiding is how you say hello to foreigners). And when you're a Kongo businessman willing to sell Africans to the Portuguese, it's way cheaper to abduct fellow Kongo than to mount an expedition into Gabon.

It is noteworthy that European slave purchasers would stir up civil wars because those would increase the number of slaves for sale- but I'm not sure I would call that the ultimate threat. For places like Nigeria, for example, there had already been almost a thousand years of Nigerians enslaving other Nigerians for sale to foreigners- and so I wouldn't call the Europeans the ultimate threat. (Indeed, until the Americas were colonized, Europeans had little use for African slaves.)