As far as I'm aware ego depletion is still largely unconfirmed. Having said that it still would seem impossible to say whether or not someone was in such a state of ego depletion that they could not have done anything else. Everyone is different and the huge variety of variables that go into determining mood makes it difficult to determine just what exactly let to an action. For example: one might come to the conclusion that as there is a gym round the corner that opens tomorrow it's acceptable to eat the chocolate one had been resisting for a while. This isn't the erosion of willpower but a rethink of the situation based on new observations.
Personally I'm skeptical of trying to measure mood in in a qualitative way liken this, not because it isn't useful but that there is a lot of scope for misuse by applying it to situations that are superficially explained this way but in reality are more complex.
Human beings are obviously limited. No matter how much willpower I have I can't be the King of England, or fly unaided, or be downstairs and upstairs at once, or jump to the moon etc etc. Similarly being strong willed, able bodied and intelligent does not give you the ability to do anything. A lot of life decisions are made for you by events outside of your control.
It's not synonymous but related.