There's alot you can do with one string when you mix in playing between open and fretted, and then adding in hammers and pick offs. Many guitar solos are done on only one string, one of my favorites is a song done by Lincoln Brewster.
Maybe I'm just in a sour mood, but that's kind of dumb. He didn't have to remove all the other strings just to not use them. Also really limits what key hey can play in.