the most important sources of our Traditional "big" ballads are the English and Scots Travellers

That's like saying most lost keys are dropped under streetlights because that's where people find them.

It doesn't support the argument at all. It says they retained them and sang them to collectors, but says nothing at all about how they were created in the first place. And almost everything collected from Travellers has also been collected elsewhere, usually many times.

David Buchan died too young to be confronted with the silliness of his argument. There are two major groups of songs that don't fit. One is ballads created by broadside publishers, maybe by versifying pre-existing folktales - these account for most of the Child corpus. Another is songs which have been reduced to fragments from a once-continuous work on an epic scale - A.L. Lloyd argued that "The Outlandish Knight" was one such, riddle songs are often in the same category (where the riddle marks a moment in a complex plot). This certainly is a Chinese-whispers process, driven by social changes that made the performance of an hours-long epic out of the question.