if i turn your question around and just say "let me eat 500g of protein a day and not workout and I will gain muscle"
that statement is just as absurd as saying you can't gain muscle by intermittent fasting.
let's not follow what professional bodybuilders do. they are a ton of drugs including HGH and insulin.. they also have top .01% genetics.. if you follow the way they eat you will end up just getting fat and having a very very short lifespan.
the best way to train if you intermittent fast is fast all day then workout.. THEN you eat in your 4-6 hour window. that is the most optimal way to do it. if you must train in the AM then that is okay too but you just have to wait until about 2pm to have your first meal.
and no just like you can't just eat 500g of protein a day and lay in bed will you gain muscle you won't gain muscle IF and not working out. get on a good workout program to gain muscle. most people at the gym have no plan or clue what they are doing so they never go anywhere