The numbers in that tweet are based on this (pre-print) paper which reports a higher rate than the CDC does, and doesn't give a figure for Moderna specifically, so doesn't really work as a source for that "known fact" if that was your intent.

I don't see how body mass and aspiration can possibly explain the presentation of vaccine associated myocarditis we're seeing. i.e. increased incidence in pubescent boys and young men. They should tend towards lower body mass than older men, and the people administering vaccines don't aspirate in general, not just for some demographics. This demographic is just generally more susceptible to myocarditis from any cause.