"What we speak of as 'traditional' is just a vignette of what people were singing when the collectors were on the rampage"

No, it isn't. 'Traditional' in this context refers to a specific body of English-language balladry and song, created and passed on under specific conditions. Sharp, etc. deliberately overlooked much of "what people were singing" (music hall, and other more recent commercial compositions) because it did not fall within their working definition of 'traditional' or 'folksong'.