You do have to take into account that uncommon breeds are very unlikely in crossbreds, and that breeds share common ancestry and therefore genetic markers.
For example a purebred rough collie may share genetic markers with similar breeds like bearded and border collies, but also with other breeds like borzoi, or large herding breeds like ovcharka. Not to mention Shetland sheepdogs, and onwards into Australian cattle dogs, Australian shepherds, the list is endless once you start. That doesn't mean that a rough collie is a cross of those breeds, but that way back when, similar dogs went into their formation.
A basenji shares genetic markers with other primitive spitz types, the most common of which in Australian breeds would be a dingo. Dingo is behind the kelpie, koolie and the Australian cattle dog. And yes, other primitive spitz types also share behavioural traits like yodelling and licking. All those genetic markers are intermixed in varying levels depending on individual dogs and different family lines.
While I think these tests are fun, I also think they are full of holes and not reliable in the least. If I had money to throw around I would love to get some done on purebreds to see what came back.