That figure is also for clinically recorded cases. Myocarditis is one of those diseases that aren't easily diagnosed by blood test/imaging. MRI can do it but only if the heart is very significantly enlarged, but they generally don't do MRI scans without good reason. So they only usually catch myocarditis when it's bad enough to create symptoms like heart failure and they start looking for causes. Which means the vast majority of mild cases go undiagnosed.

That goes for vaccine induced myocarditis. The ones who end up in hospital and get diagnosed are at serious risk, but there's nothing to say those who got the vaccine and experienced some mild, subclinical inflammation are also at that same level of risk.