Huh? Not following your logic.
Since Covid itself is known to cause myocarditis, the study needs to include at least four control groups, sampled during the same time period, with similar demographics: (1) People who never had Covid or the vaxx; (2) people who had Covid, but not the vaxx; (3) people who had the vaxx, but not Covid; and (4) people who had both Covid and the vaxx.
How do we know that these heart issues are not simply a side effect of having Covid ? It’s hard to distinguish whether it’s the vax, or Covid that’s causing this… since the vast majority of people had both (vax and Covid)