This statement is a half truth used as an excuse to avoid attempting anything difficult. (No one ever said crash dieting was easy.) It is true that you will lose water weight when calorie restricting - because lowering calorie intake forces your body to turn to glycogen stores for more energy. Each gram of glycogen carries with it 3 grams of water, so as those stores are depleted, a lot of water leaves with it. Afterwards your body doesn't turn to muscle for more energy, it turns to fat - this is explained in point 4.

On the other hand, if you go water fasting for a month (yes, people do that) then you will lose some muscle mass. But that isn't what is being referred to as a crash diet, so not sure why I even mentioned it. Besides, most of us have enough fat stores to not need to worry about this 🙊