"If it's claimed to go back beyond grandma (assuming today's singer really learned it from her)..." [my italics]

So are we to presume that singers' own testimony about where they learned their songs is now to be regarded, by default, with suspicion? Obviously we know a lot more now about the role of print in propagating these songs, but just about every singer who was ever asked the question has stated that they learned the bulk of their songs from family members. Why would they choose to fib, or be somehow mistaken, about having learned a song from grandma? There's a big danger in ignoring such direct evidence as we have, just because it might be inconvenient for prevailing orthodoxy.