"If you can break it down to one voice and an acoustic guitar, it's folk."

Sorry, but this is way too simplistic. As others have already pointed out, the piano can be just as folkloric an instrument as anything else.

I'd still like to take issue with the idea that folk music has to be simplistic. Tell that to your average Bulgarian folk musician turning out dance tunes in 15/16 or similar. The idea of it being linked to rusticity and simplicity has its roots in all that Herder/Grimm noble savage stuff.