Comparing these two numbers is completely wrong. The study that you cite performed an ECG on 5000 high school students after a COVID-19 vaccine and found 50 had abnormal ECG. But having an abnormal finding on an ECG doesn't actually mean that there's any underlying heart condition. I just went through this myself - I had a bunch of ECGs for a clinical trial (unrelated to any cardiac issue). One came back abnormal. Repeating the ECG in my PCP's office showed nothing. A cardiac stress test showed nothing. About 10 other ECGs for the clinical trial showed nothing. It was just an incidental finding. Saying that 1% have "cardiac damage" because there's an abnormal ECG is just completely wrong. The test has a false positive rate that's greater than 0. And 50 abnormal findings does not mean 50 cases of "cardiac damage". In fact there were 5 - 1 myocarditis and 4 arrhythmias. All of these cleared up on their own.
And finally, you can't compare the two studies because they are looking at fundamentally different things. The 3 excess cases per 100,000 doses comes from looking at millions and millions of health records, so it will only show cases that were actually diagnosed in the real world. The paper you cite performed an ECG on everyone in the study - so of course they are going to find vastly more cases, because they are doing vastly more testing. But that study is not performing ECGs on anyone who gets COVID but has not been vaccinated. If you did that, you would also see myocarditis, because viral infection is the leading cause of myocarditis.
You cannot conclude anything from the study that you cite about the relative cardiac risks of the vaccine - it's just not a study that's designed to do that.
And finally, you can't compare the two studies because they are looking at fundamentally different things. The 3 excess cases per 100,000 doses comes from looking at millions and millions of health records, so it will only show cases that were actually diagnosed in the real world. The paper you cite performed an ECG on everyone in the study - so of course they are going to find vastly more cases, because they are doing vastly more testing. But that study is not performing ECGs on anyone who gets COVID but has not been vaccinated. If you did that, you would also see myocarditis, because viral infection is the leading cause of myocarditis.
You cannot conclude anything from the study that you cite about the relative cardiac risks of the vaccine - it's just not a study that's designed to do that.