Well, I stand corrected. I hadn't given any thought to how relatively "new" an instrument the present-day guitar is.

Nevertheless, I'm glad I put forth my misguided bit of speculation, if only to have prompted further discussion.

One statement with which I'd like to take issue:

Piedmont sounds like two instruments when played on one guitar.

Yeah, true enough ~ but the same can be said of quite a few other styles/traditions where the thumb plays in a steady rhythm on the bass strings while one, two, or three other fingers play more-or-less independent patterns and/or melody lines on the treble strings. Besides the Delta blues style that emerged in North Mississippi, that approach is seen in country music out of the white Appalachian community ~ e.g., "Travis" picking and Kentucky-style "thumbpicking."

It may well be true that this approach originated in the Piedmont region, since that area was settled by non-natives (Europeans and Africans) earlier than points further west. It's still impossible to know if what we describe today as "Piedmont style" is the original "sounds-like-two-instruments" method, or if it's simply one of several different subgenres all descended from some earlier playing style.

One thing is pretty much for sure: whoever first developed this highly sophisticated approach to guitar playing was absolutely an American folk musician, working/playing far outside the music establishment of conservatories, concert promotion, etc., and probably (though not necesarily) African-American.

I can't provide a citation, but I've read more than once about Andres Segovia's reaction to hearing a recording of this kind of solo guitar playing. (If I remember correctly, he was listening to Mississippi John Hurt himself, although I'm less sure about that part of the story.) The highly talented and deservedly renowned Segovia assumed he was listening to two players, and was dumbfounded when told that the sound was the work of a single player. If this old story is true, or even partly true, it's a good indication that centuries of European-based musical study and tradition had never come up with anything remotely like the folk-guitar fingerpicking that so many of us take for granted today.