It is a really dumb guidance by the surgeon general.
It is using a statistical correlation to infer a heightened death risk using only death certificate data for cardiac related causes of death. It is relying on the window after the second dose of the vaccine to infer causation regardless of if it makes sense (the ICD codes used were a catch-all of cardiac codes). And the method doesn't rely on a comparator group of unvaccinated, but comparing the window to the overall period examined for the vaccinated group in question. All cause mortality was not increased which should be your first clue something is not right, only a catch-all of cardiac codes on the death certificates. And the sample size for the heightened risk was very tiny.
It is unclear as well if they were using presence of the cardiac ICD code on the death certificate or limiting it only to the underlying causes of death. Death certificates will list the immediate cause of death, intermediate causes of death, and finally the underlying cause of death, and will also list contributing conditions to the death that were not related to the condition causing it.
Further in making the guidance the surgeon general did not actually weigh the costs and benefits of vaccination for the age group in question. Even if we assumed this risk was real, the benefits of vaccination are significant. The CDC actually has weighed the costs and benefits of vaccination and found the benefits far exceed the costs for all age groups.
The CDC actually investigates all deaths reported to VAERS analyzing medical records, and autopsies if available, to actually determine causation, so this sort of inference is less sophisticated than what we already have done. We've had no deaths from the mRNA vaccines, and 9 with the J&J vaccine from TTS so far.
And yes mRNA vaccines carry a rare risk of myocarditis and pericarditis, but this risk is self-resolving and people have a full recovery within a few months. In contrast coronavirus infection itself carries its own higher risk of cardiac-related issues that are more severe and with worse long-term prognoses.