Perhaps your concept of "traditional" or "folk" is simply irrelevant to what Roud is investigating? He's looking at the songs people actually sang, and the social praxis of singing.
Roud deliberately chose Loyd's title for his book, indicating they are on the same subject - they are not
That is misleading

If you're buying a book that big and expensive, you read the cover blurb, the foreword and maybe a review.

I work in a charity bookshop and have to deal with literal-minded volunteers who file The Photoshop Bible in the religion section. The title doesn't mean Adobe claim to be a religion. (The same people would put both The Secret Garden and The Perfumed Garden next to Alan Titchmarsh). Titles don't matter if you have a clue.