I hadn't heard of it before. That sounds like a very bad idea. How long do you do it for? If you did it for too long you'd be risking vitamin deficiencies, possibly scurvy. Not to mention how boring it would be! Potatoes do contain protein though (pretty much all food does), just not a lot by weight.
I think I'd never eat another potato in my life if I did this diet. No thanks. I like my food too much. I recall watching a interview with the chap who did this diet and he looked okay. Must have had great willpower.