think the author must be very Eurocentric and indeed likely some kind of western supremacist.

While elevating himself to the position of global perspective and non-European centric view.

Also, he's specifically talking about places like Cape Verde and then in the Americas, where there were no established authorities to put a check on the extent of the slavery.

I don't know the history of slavery, is that a meaningful distinction? Were African tribes selling conquered peoples to the European merchants nicer to their goods?