He probably wouldn't have been able to pull that off without the open tuning, though. I even think he may have a couple of identical strings on there, tuned to the exact same pitch. You can hear the subtle phasing/flanging, just as with the double plain strings on a twelve-string guitar.
Don't get me wrong, though. I do think it's impressive. But he's also not the first to do that sort of thing. Run a search on Kaki King, who was already doing the same sort of thing ─ but with songs of her own ─ more than fifteen years ago. ;)