I don't know where else. But I do know word travels and has traveled since the beginning of time. Early African Christianity was confined mostly to the North of Africa, yes, but it's not completely unreasonable to assume that there were some Christians dispersed all over Africa, especially when you consider just how word travels. After all, many African civilizations were built borrowing technologies from each other. Obviously word traveled in those cases, so why couldn't it have traveled in this scenario?
The only reason why I made note of Christianity existing in Africa before European invasion is so that people can understand that Christianity has never been the "white man's" religion. Africa has always had knowledge of it, which makes the quote too much of a reach for me to take seriously.
So basically you and I don't know for sure that Christianity was anywhere else in Africa than those places.
Now I understand your point that Christianity is not "the White man's religion" (which should be obvious to all since it was born in the Middle-East, but got so white-washed since taht people tend to "forget" that detail) but just "assuming" that Africans knew about Christianity because of "word of mouth" is not very solid for me.
Until evidence to the contrary, I can't believe Africans in what is now Ghana, Congo, Botswana, Nigeria etc knew of Christianity before it was imposed upon them by missionaries and colonization. If it was known, it was most probably very marginal, maybe some populations on the Nile river and some pockets in Eastern Africa due to the proximity with Ethiopia. Hell MAYBE word did travel and Africans looked at Christianity like
and didn't mess with it until it was imposed by Europeans. There's just no evidence (churches, bibles, anything) of long standing Christian presence anywhere outside of Northern Africa and Ethiopia that I know of.