this is a topic which is near and dear to my heart, literally, as one of the fraction of a percent of people for whom extreme cardio is not just “possibly bad” but demonstrably catastrophically bad. the science around my condition is evolving, and in the short time (9 years) since I was diagnosed, the medical understanding of it has changed greatly. In my case, genetic mutations (dozens have been identified) cause the heart to develop scars and fatty inclusions during what would normally be the healing/recovery/strengthening period after extreme stress on the heart muscle. those scars or fatty bits then disrupt the electrical system that makes the heart beat regularly, resulting in anything from PVCs (annoying in small quantities, harmful and debilitating in large number) to ventricular tachycardia to ventricular fibrillation and sudden cardiac death. the irony is that if you even if you have the genes, never exercised hard, your heart would never develop the issues.
what has changed most in the 9 years is the broadening of the understanding of this particular cardiomyopathy from one caused by a very few mutations and mostly affecting the electrical workings of the heart and almost entirely only one chamber of the heart to a range of related degrees of disease causes by dozens of different mutations and often affecting the heart much more broadly. nonetheless, the first indicator, partilcularly in young people, is often death.
the article and discussion is interesting, but alongside the unknowns about the effect of a ton of exercise on normal hearts, it is worth noting or reminding everyone that if your heart rhythm is irregular or otherwise abnormal (typically accompanied by light headedness, fainting, a feeling of pressure in the neck, anxiousness, palpitations, sweating, etc) you should get a referral to an electrophysiologist ASAP. I was 36, immediately got an ICD, meds, two emergency open heart surgeries at 37, multiple defibrillations and hospitalizations and procedures and new meds and fainting and medical drama of many kinds.