Lots of guessing in that statement.
“Independently from vaccines, myocarditis occurs in roughly 23 per 100,000 peopleworldwide per year (we don’t have reliable figures for pericarditis). This shows us there’s a much lower risk from vaccination than exists in the population generally.”
Not guessing, just how medicine is, it's not absolute. I'm definitely not anti vax, but just pondering why this case could be different.
In regard to Myocarditis, mild cases would probably not even be known unless you present symptoms. So the difference of numbers between diagnosed and undiagnosed would be very high. The study founded diagnosed acute Myocarditis was found in 0.9% of covid recovered males aged between 12 and 18 and the rate of vaccine related was surmised at 1/6th of that. To diagnose this you would need x-ray, echocardiogram, MRI or such and means the patient would most likely have to display symptoms without being admitted and these procedures are costly if not paid for by the study. ECG could detect abnormal rythym but that's not guaranteed. The younger folk may never know they had a case of this until they get some other issue and find scarring on the heart muscle. A bit of shortness of breath or tiredness whilst already known to be fighting a virus, you could easily over look it.
Last but important point, Myocarditis is a result or reaction to having a virus, so first you have to be in the under 30 group it mostly affects, then have a particular virus that aggravates the heart and then be in the group susceptible to this condition. So the numbers are not going to be huge, you would hope. If they were Covid would be killing young people very efficiently.