This is a crazy argument. So the shepherds that you see on classical Greek Vases- the ones who played the double pipes, never played two notes at once?
So Pythagoras, who explained the mathematical basis of harmony, had never heard a lyre player hit two or three strings at the same time?
Tomb paintings of Egyptians playing shoulder harps with both hands were only playing single note lines?
I cannot believe that musicians have changed so much. the phrase "Hey these two sound great together!" must have been heard long before the dawn of written music history- in every language on the planet.
I'd suspect that finding harmonies was discovered long before the phrase "that's not folk" was first uttered.(probably in Sanscrit)
Cheers
Dave