Obviously a potato farmer would eat the smaller ones that don't make the cut for grocery store bags. Healthier though is questionable. You can fast and temporarily improve some vitals and lose a few lbs but you're decreasing your body's nutrient stores and some of that weight loss is bound to be muscle.
It isn't a sustainable diet so I see little useful in his doing it. We already knew that cutting down calories and fat would benefit anyone who eats an excessive amount of either, but you still need a balanced diet.
What were the "naysayers" saying exactly? That he'd die? That wouldn't be expected in 20 days. A reasonably healthy person can eat nothing and survive that long as long as they have water. That he'd hate potatoes from then on? Maybe!