In that case the shot is highly unlikely to be a significant factor, seeing as how exercise-related myocarditis has been a thing since long before COVID was a t

In that case the shot is highly unlikely to be a significant factor, seeing as how exercise-related myocarditis has been a thing since long before COVID was a thing. Demonstrating the vaccine to be a factor would require checking for myocarditis before and after vaccination, and also doing so for the control group. I don't know of any studies which do that, though; usually it's more the other way around: diagnosing myocarditis and correlating that with a past vaccination or infection.