There is no control group for unvaccinated people in this study. Everyone who got myocarditis from COVID had at least one dose of the vaccine. So the correct statement is "COVID is between 7 and 40 times more likely to cause myocarditis IF you have gotten at least one dose of the vaccine"
If you want to argue against the article I suggest considering the bias of being 1 year older after the exposure than before it. For a large population that might matter for myocarditis incidence. I suspect they have corrected for this however.
You are free to challenge the latter part, but so far there is no evidence that the vaccines "stay in your body" or something similar.
Amongst those with at least one dose, there were 3,028,867 (7.8%) individuals who had a SARS-CoV-2 positive test. Of these, 2,315,669 (6.0%) individuals tested positive before vaccination; while 713,198 (1.8%) and 298,315 (0.7%) tested positive after the first and second vaccine doses, respectively.
If these effects take 1 year to manifest, then it’s hard to identify the causal factor with this setup
However, while very hand-wavy on my part, I believe there's plenty of evidence that Covid on its own wreaks havoc on our cardiovascular system, so I'm definitely leaning towards it being the biggest sole risk here.