Folk musicians do use sixths and ninths,,Elizabeth Cotton in Wilson Rag,most blues musicians,and some traditional muscians ,myself included, certainly use them,.
I also use dominant elevenths, sometimes as a partial substutution for dominant seventh, If it fits harmonically with the melody.
   no one music is an island unto itself,there are always crossover points where jazz influence creeps into traditional music, listen to shetland fiddle tunes accompanied by guitarist Willie Johnson.
he uses jazzy 6 9 11 chords, and it sounds great.Dick Miles