I think willpower is a description of the human experience. I don't know if it's a good mechanistic description or not. It does not seem to be taken seriously as a cause by many psychologists.

Whatever you think of willpower, I think controlling the food environment (don't bring it home from the store, don't eat at restaurants with mainly junk food, etc) is the most effective intervention. After that, things like substitution, delay, smallest possible portion, and putting the tempting food in an inconvenient location may help.

Psychology research is all over the place. If they mean willpower as this general concept, then probably it's not a thing.

But if you have junk food in your home some people (like myself) will eat it compulsively at some point.