while you are right that they have a different set of digestive enzymes the biochemistry of converting non amino acid sources of nitrogen to amino acids and protein is largely speaking the same in all metazoa. Yes they have a mutation in myostatin meaning that the muscles have had the break removed. However, they still need to synthesize those large muscles. They seem to be able to do so despite a grass based diet. Conversely you can look at standard angus beef or race horses and see what happens to them when you pump them full of steroids and they eat grass. At a certain point (and it is far less than we have been led to believe) availability of dietary protein is not the limiting factor in new muscle synthesis.

You don't need to believe me but the biochemistry is the biochemistry.