Neapolitan

"Folk Music" in the original sense was music unique to a community because it was isolated, it was passed down from person to person. "Folk" became an umbrella term for anything that sound like Country song using acoustic instruments, Appalachian Folk song, and 50's & 60's American Folk Music Revival. Because American Folk Music Revival is known as Folk, Traditional Music is now instead .

The world was different place in the 19th century, which allowed Folk Music (Tradition Music) to form, those elements don't exist today, the critea that defined Folk Music (Tradition Music) is gone. Popular Songs, today, are recorded and writen down, the songs are copy-writed; which is the opposite of how "Folk Music" was defined.

Every country has it own folk music, that is unique to that country. Music in America that met the critea of Folk was music from Appalachia. What is considered Folk Muisc today is really American Folk Music Revival. Started by Pete Seger and made popular through artist and groups in the 50's and 60's,
like the Weavers, the Kingston Trio, and especially Bob Dylan.

So artist like the one you mentioned are really playing a Folk-style of Popular Music not really Folk or Traditional Music.