I think the point here is that the article should have said that Covid is between 7 and 40 times more likely to cause Myocarditis _for vaccinated people_ because unvaccinated people weren't studied.
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.