Is it possible to gain muscle tissue without exersicing first?

In recovery from anorexia with no exercise, will I not gain muscle tissue before I can be more active (when the healing is done)? I read Matt write that we gain fat in our muscles in recovery. Why is that?

And would the end result (of the body composition) be different if we gain on mostly fat-calories instead of carb-calories? I’m thinking about Matt saying the metabolism is improved more by eating mostly carbs compared to eating mostly fat.