Wikipedia says that mice are herbivores, eating primarily fruit and grains, FWIW. This jives with what I've always thought about their diets, not very high protein. I think it's a real stretch to compare a diet that may give a mouse more longevity (or most any result), with a possible human diet. Rats are probably a better match, but I still think there are innate differences of all kinds in the "ideal diet" for all species.

If you throw non-grain seeds into the mix, the protein content probably goes up some. Anyways, the protein-lowers-lifespan isn't consistent, even in mice.