I think that's about the strangest statement I've come across in a long time

I was just about to say the very same thing.

My wife and I under another Folk nom de plume regularly sing "I will", Bob Fox does a nice rendition of "from me to you", John Williams an instrumental version of "here comes the sun" (also a nice version was to be heard on "The flip side of Dominic Hyde"), Norwegian Wood works well in DADGAD etc.

And if you want to expand Bluegrass into the "Folk" category listen the The Charles River Valley Boys LP/CD "Beatle Country"

PB