I don't think it's so much that he can play any stringed instrument, but more that he can apply what he's learned from guitar (including weird tunings) onto another instrument. Like most of 'em are basically played the same as a bass or guitar with the exception of the berimbau, cello and harp. I believe he said in his most recent video with Davie504 that he takes like half an hour to learn the basics of a new instrument and go from there.
From the sounds of it, pretty much any moderately experienced guitarist or bassist can learn those other instuments like the banjo, mandolin, or sitar.. and that's pretty dope! I wanna see him do a personally wanna see him do a video on the Japanese koto since it's such intricate playing style of a stringed instrument~