> We did not observe an increased incidence of neither pericarditis nor myocarditis in adult patients recovering from COVID-19 infection.
you should be careful not to interpret this as evidence of absence. The previous sentence gives confidence intervals:
> Post COVID-19 infection was not associated with either myocarditis (aHR 1.08; 95% CI 0.45 to 2.56) or pericarditis (aHR 0.53; 95% CI 0.25 to 1.13).
Here a 95% CI from 0.45 to 2.56 means that the data is statistically consistent with myocarditis being 0.45 times as likely in people with COVID infection, or 2.56 times more likely.
The sample size here is simply not enough to be more precise, simply because myocarditis and pericarditis are so rare. (It's hard to be precise about the rate of something that happened in only nine COVID patients in the study!) Hence you should not interpret this as demonstrating no difference between people with and without prior COVID infection -- merely bounding the size of the difference that may exist, and saying it can't be more than roughly a factor of 2.5-3.
To be blunt, the authors are wrong to say in their Conclusion that "Our data suggest that there is no increase in the incidence of myocarditis and pericarditis in COVID-19 recovered patients compared to uninfected matched controls." Again, the confidence interval shows that a 2x increased incidence of myocarditis would be consistent with the data.
This is a widespread misinterpretation of statistics and one that has been around for decades, so please excuse me for taking every opportunity to complain. (e.g. https://www.statisticsdonewrong.com/power.html#the-power-of-...)