I'm pretty sure he's talking about this paper
This cohort study evaluates the presence of myocardial injury in unselected patients recently recovered from coronavirus disease 2019 (COVID-19).
jamanetwork.com
I think the primary criticism he's probably hinting at is that the study is not of college aged COVID patients.
He also seems to be hinting that other viruses can cause myocardiial inflammation.
No matter what he's intending to argue; he's falsely characterizing the cancellation decision as solely being based on that paper. Additionally, if he's not directly citing the study and doesn't lay out his argument, it's a pretty good indication it's a haphazardly formed argument.