well, ofc it is not suitable for 120kg+ guys that are in good shape and have tons of muscle. They wont be able to get their caloric intake for the day in the feast window.
But for an "average" gym rat (natural or not doesnt matter in my opinion) it does not make a difference. I can tell you from first hand:
I always used to eat 6+ meals a day, even marco distribution. Compared to IF, i put on fat much more easily without adding extra muscle. (bf measurements proved this, and not with a bf scale, i dont know the english word for the measurement instrument, it measures thickness of your body layers). Now on IF, i still put on some fat along with muscles but way better ratio.
I dont believe ANYONE loses muscle during 20hours fasting. For pros on the chase for maximum mass there are other factors that make IF unsuitable
(amount of calories, going hypo due to drugs, ...).
BUt an average gym rat like me who just needs 3000kcal a day to lean bulk there is no disadvantage (yet, maybe i will find one if i am 1+year on IF)