The claim here is that carnism is necessary for peak athletic performance, and therefore is justified
Put down the Doritos. This isn't you. Every time I enter into a discussion with people about top athletic performance I get the sensation as though they feel it somehow trickles down to them when they haven't lifted a weight in their life.
Plenty of research shows the bioavailability of nutrients and performance of athletes is not tied to athletic performance. For more information consult the film Game Changers.
Is pure peak athletic performance worth killing animals for? "Let's see how much one man can deadlift. To really test this out, we'll need to kill some animals." This is not sound reasoning. Generally carnists tend to agree that dog fighting is immoral even if it's somehow the "rawest" or "best" form of entertainment in some category, and that seeing the best-fighting dog isn't worth the suffering imposed on these animals.