Choral singing when applied across the board to any type of music (in this case folk) is not only 21st century, but is confined only to the revival as far as I can see

Choral singing - and in particular, polyphonic choral singing - dates back thousands of years and has always been the norm for a large fraction of humanity. Lots of good examples linked from here:

http://music000001.blogspot.co.uk/

There are many musical cultures where solo singing is unknown - all performance is in polyphonic groups.