How well does this stack up against the milk-and-potatoes diet?

According to the popular science/history book 1493, it is more-or-less a complete diet in itself. The two datapoints it uses to back this up are a story about two Polish scientists who ate nothing but potatoes and milk (or milk products, like cheese and butter) for a year, and remained in perfect heath for the duration, and the claim that prior to the Potato Famine, 25% of the Irish population was already surviving on nothing but milk and potatoes indefinitely, and they were regarded as a healthy and hale race.

A 2000 calorie diet of milk and potatoes (4 cups 2% milk, 5.5 300g potatoes) costs around $65 / day. Preparation time involves as little as microwaving a potato and pouring a glass of milk.