I agree that there are plenty of possible reasons for poor personal performance, and it would be foolish to assume that more sleep/exercise is a universal solution.
But the article never even went there. It's specifically talking about willpower - getting it back or getting more of it. I'd say that the concept of willpower is more of a subcategory to the larger idea you mentioned about underperformance. In that sense, it's about solving a piece of the problem, not the whole thing.
