What's interesting there is the technical matter of not damping those low strings. They ring very nicely, and he wants to let the ring, so he has to make sure the right hand and forearm don't foul those strings.
This sort of thing is not that new. Instruments with unfretted bass strings or sympathetic strings have been around for a while. Lots of variations on that theme.
Sting and his lute
John McLaughlin had a custom Gibson with sympathetic strings - which were still open to the right hand
Pat Metheny's "pikasso" is a multi necked instrument, but also has courses of unfretted strings that he can strum or pick or just allow to resonate
The Coral electric sitar that was popular in the late 60s and early 70s (listen to Denny Dias's solo on Steely Dan's "Do It Again") had sympathetic strings (as well as bridge designed to buzz)
The idea goes back to the 17th century at least, it didn't start in the 60s when everybody took lots of drugs ? This is Lynda Sayce, who plays a
Theorbo. Her instrument is contemporary in build, but based on an instruments that survive in various museums.
