That is also not the full article and does not explain about the proposed underlying cause. 🙃
Discussion
I think it's pretty clear findings without reviewing the supplemental materials;
"A notable finding was that markedly elevated levels of full-length spike protein (33.9±22.4 pg/mL), unbound by antibodies, were detected in the plasma of individuals with postvaccine myocarditis, whereas no free spike was detected in asymptomatic vaccinated control subjects (unpaired t test; P<0.0001)."