My two cents please; a 25 year reenactor with an "authentic" musician impression.
Yes, Jon comes on strong and loves to talk about how much he knows(a little too much knowledge that is dangerous), but when you get past that you will see his main point. What most people believe to be old music is actually quite modern. Jon's intent is to improve the scolarship of the subject which is one of the excellent by-products of reenacting.
Some more perspective. Forget the "they had it, they'd have used it" mentality (dulcimers, steel strings, mandolins, etc.), it was, as today, a question of fashion, what was common, what was rare, what was the trendy thing. Are you likely to see an oboe in a rock'n'roll band? No, and this is one of Jon's points. And too many do what is fun and simple for them and ignore the thill of the search, the discovery and the opportunity to learn, to teach and enlighten.
I don't buy the "can't afford it" line. Period instruments are no more expensive than your musket or your Yamaha. Banjos are fun to make and not that difficult.
I do fing some photos of soldiers with guitars, but more with banjos, which was the hip instrument. Leave the steel strings and picks at home along with the mandolins. Although harmonicas were but a novelty, a toy, at least take the covers off them so they resemble those as they were made then.
And if all you guys who complain about the authenticity nazis had just taken up the fiddle I would have been a happy guy long ago. If I never hear another Neil Young tune around the campfire my pleasure would be complete.
Some roots of folk music education is good for the Mudcat crew. Too little attention is paid to much of our fine musical tradition if it's pre-Lomax or not on CD or tape. After all the first indiginous American art was not Jazz, but Minstrel!