That doesn't mean there weren't previously undetected cardiac problems. Many, many heart conditions don't have symptoms and don't manifest until there's a sudden MI or arrhythmia.
In this case Boy A had scarring of the heart that likely led to the arrhythmia:
"One might expect some scarring/repair after a few weeks, although the scarring in Case A appears more organized than the three-week interval between the vaccine doses. Also, it is only in one of the cases. It remains possible that the fibrosis represents arrhythmogenic cardiomyopathy."
Arrythmogenic cardiomyopathy is an inherited, genetic condition.
Boy B clearly had a condition, cardiac hypertrophy, that's most frequently inherited but can also result from long term stresses on the heart such as high blood pressure. It's a thickening and enlargement (not inflammation) of the heart muscle that can also result in arrhythmias. Regardless of the cause it's a longer term process:
"...the cardiac hypertrophy in Case B may have made the heart more susceptible to anarrhythmia. The key point is that since these boys had died suddenly and unexpectedly in their sleep without resuscitation, if the arrhythmia had been due to the myocardial scar (Boy A) or cardiomegaly (Boy B), then the fulminant, global myocardial injury would not be an expected finding. These two clinical histories support the etiology of the acute myocardial injury as a primary factor not a secondary agonal or post-resuscitative artefact."
The vaccine-related myocarditis may well have triggered the arrhythmias here, I don't know.
What I do know is both boys had underlying cardiac conditions and the report doesn't blame either of their deaths on the vaccine or myocarditis.
It’s teenagers to mid 20s with sports players that unknown cardiac issues are discovered.
The older the child (late teens early 20s) the harder they train and harder they play.
You’re right about that.