The study only used VAERS database to estimate the incidence of post-mRNA vaccine myocarditis. There's no control or reference group, however.

It's true that underestimation is a major limitation of VAERS, so it's given that the myocarditis rate in this study is likely underestimated. However, another main limitation of VAERS is no control group, so background incidence or incidence in unvaccinated group is missing.

Plus, I think that study has a bright side. 98% of the post-mRNA vaccine myocarditis cases were mild. Intensive therapies were only applied to 14 out of 676 cases (12 required vasoactive medications and 2 required intubation or mechanical ventilation).