I do see what you’re saying, but perhaps I should be more clear on the data. The “risk factor matched controls” are not those who had similar respiratory infections but rather people who had similar comorbidities like heart disease and hypertension which can cause similar findings of myocardial abnormalities.
They compared three groups: healthy people without comorbidities who never had COVID, the aforementioned risk factor matched controls who also never had COVID, and a group of people who had COVID and had similar cardiac risk factors (heart disease, blood pressure etc.) to the matches controls.
Ultimately, the healthy group had significantly fewer abnormalities while the risk factor matched control and COVID + risk factor were not significantly different. Hence, the risk factors caused the abnormalities, not the COVID.
This is only one study on people who did not have sever COVID and we have case reports of myocarditis. However, this study does cast a shadow on the idea that there will be some epidemic of myocarditis in people who had mild or asymptomatic COVID.