The 1 in 1000 figure comes from a study where they did an ECG on 5000 high school students. They found one case of myocarditis and four cases of arrhythmia, all of which improved on their own. You can't compare that 0.1% figure with figures from studies that look at millions of people and see how many were diagnosed with myocarditis, because the act of giving everyone an ECG will lead to diagnosing more cases of myocarditis. The proper comparison is how many cases of myocarditis and arrhythmia would they have found if they did an ECG on 5000 high school students who got COVID-19 without being vaccinated. And the answer to that is they would have found at least a comparable number of cases because viral infection is the most common cause of myocarditis and because our population-level studies show excess cases of myocarditis after COVID-19 infection in unvaccinated people.