It sounds like this is a risk for dogs/breeds that have the mutant MDR1 gene, but not a severe risk. I'm not aware of Basenjis having this problem. Someone correct me if I am wrong.
I have not heard of a problem specifically with basenjis…and they aren't named on the Heartgard Plus website (which was where I read the warning originally). Plenty of people use Ivermectin on their dogs just fine.