Lighter wrote:

Who would have seen through him? A handful of academics?

Well, exactly. :-) This doesn't affect his performances; those would stand or fall on their own. And even scholars would be unlikely to notice what he did in a performance, since they wouldn't have every text of every ballad in their heads.

The curiosity is the Ballad Book. Since it's explicitly devoted to Child Ballads, it is aimed specifically, although probably not exclusively, at the scholarly community -- ordinary people wouldn't care that a particular song is a Child Ballad or not. (Even I don't care about that, in a performance -- I only care that it's a good song.) So, basically, he was saying, "Here are my fakes; expose me!"