People are more likely to overtrain or undertrain than to under eat. Muscles grow outside of the gym especially when you’re sleeping. So if you don’t allow them sufficient rest time to grow, then you start to get weaker. Also remember that your muscles can still grow even in a slight calorie deficit. Not ideal but it’s possible. So it wouldn’t make sense for overeating to be the key. The reason it’s a common belief is because people are looking at bodybuilders on steroids and how they train. That doesn’t apply to natural lifters.