Medlife crisis, a cardiologist youtuber has recently made a video [0] about a similar study, that also looked at MRI images of patients [1]. That same story is referenced in the article as well. According to medlife crisis, MRI images aren't really helpful as a sole tool to diagnose miocarditis, instead one uses them together with other data like symptom reports or ECG. Alone, they would be too flaky. Also, we don't even know how healthy people look like as we don't put them into scanners.
Now this [2] study, the one the blog post is about, didn't just study MRI images but also collected other data, like ECG. But quoting the study:
> There were no diagnostic ST/T wave changes on electrocardiogram, and ventricular volumes and function were within the normal range in all athletes by transthoracic echocardiogram and CMR imaging. No athlete had elevated serum levels of troponin I.
Some individuals reported mild symptoms typical to COVID, but most were asymptomatic. So the study confirmed that there are no abnormal changes beyond the MRI scans, which, as written above, can be flaky.
[0]: https://www.youtube.com/watch?v=iaiQGJqZ6Y8