To try and be a bit more specific: surely "popular" is used in the ICTM formulation in the sense in which Child used it in the title of his collection ~~ which, of course, was a term preferred to "Folk" even after W.J. Thoms's coinage of that term in 1846. Others use the term "classical", as you know, to refer to and define the sort of ballads that Child saw fit to include. It seems to me that there is an ambiguity, or an oxymoron {"traditional/popular/classical"}, within the ICTM definition which you and I interpret differently.

~Michael~