It is unclear the origin of Fanconi Syndrome as prior to the internt - keeping track of the health issues was just not as easy as it is now. There is a school of thought that it might have been originially a mutated gene perhaps from environmental toxins. I don't know as anyone has ever been able to know that for sure - if there ever is a genetic marker - that answer may come out of the research related to that.
I would also say that the gene pool being limited to start with and then further limited when some other diseases showed up (HA & PRA for example). When those were identified there was no knowledge that HA could be so easily bred around and PRA was mis-or over - identified in the beginning because of the basenji hyper-reflective eye that LOOKS like PRA but is not. A large number of dogs was eliminated from the gene pool because of that. Now that we have a bigger knowledge base and testing and protocols for these, fanconi's and other health issues, we can look at the individual African dogs with knowledge of their actual health rather than in the earlier imports just assuming that if they are from African they are healthier.
With the combination of turmoil and war in Africa as well as previously isolated areas now being more accessiblt - it is important to acquire additional genetics while they are still largely not influenced by other dog breeds. We can now do this with some knowledge and science to make sure we are not adding the same things we are trying to avoid.
Obviously others have their opinions or fine points but this is the general overview.