If this were the cause, I would expect to see similar rates of myocarditis from the viral vector vaccines, but it's much lower. The MRNA vaccines work by causing cells to produce antigens, which are presented on the cell surface and induce the immune system to attack the cell. We know now as well that the lipid vehicle doesn't stay at the injection site but distributes through the body to varying degrees. No need to posit a secondary mechanism - it's more likely the vaccine is just carried to the heart in some cases, is absorbed, and induces immune attack like it does in other tissue.

The biodistribution of the viral vector vaccines is likely different and also dependant on what tissue the vector prefers to infect, so when it leaks out of the injection site you get side effects in other places (clotting and vascular effects) instead of the heart.