I have a couple of concerns here - 1 you were told the Grandparents were 'free of fanconi' and the parents were not old enough to test. Now you have seen that they are not listed on the OFFA site and others have said that the test can be done on 2 week old puppies.
The question here is that the relationship that you have with your breeder is perhaps the most important connection that you can have in your journey with basenjis - I'm reminded of that as today I opened a Christmas card from the breeder/mentor of my second basenjis CH Maikai's Here's the Beef. I met them in 1983. Kathy helped me so much in learning how to show, helping me select a stud dog - helping raise and place puppies - every dumb question I had about basenji behavior - she helped me through it.
I try to have that kind of relationship with those who have a dog from me. I always have told them that they need to make sure that they like me as well as the dog because we are a package.
That said - the breeders of your potential puppy has told you a lie. They clearly either did it on purpose - OR because they didn't know - in either case - will they be there for you to give you advice that is specific to their lines? And can you believe it?
Obviously your decision but - with that big a red flag - I know what I would do.