Is the claim true that excessive running as mentioned, can be damaging to the myocardium and coronary arteries?
The anecdotal article you read refers to a Mayo Clinic article whose abstract is
A routine of regular exercise is highly effective for prevention and treatment of many common chronic diseases and improves cardiovascular (CV) health and longevity. However, long-term excessive endurance exercise may induce pathologic structural remodeling of the heart and large arteries. Emerging data suggest that chronic training for and competing in extreme endurance events such as marathons, ultramarathons, ironman distance triathlons, and very long distance bicycle races, can cause transient acute volume overload of the atria and right ventricle, with transient reductions in right ventricular ejection fraction and elevations of cardiac biomarkers, all of which return to normal within 1 week. Over months to years of repetitive injury, this process, in some individuals, may lead to patchy myocardial fibrosis, particularly in the atria, interventricular septum, and right ventricle, creating a substrate for atrial and ventricular arrhythmias. Additionally, long-term excessive sustained exercise may be associated with coronary artery calcification, diastolic dysfunction, and large-artery wall stiffening. However, this concept is still hypothetical and there is some inconsistency in the reported findings. Furthermore, lifelong vigorous exercisers generally have low mortality rates and excellent functional capacity. Notwithstanding, the hypothesis that long-term excessive endurance exercise may induce adverse CV remodeling warrants further investigation to identify at-risk individuals and formulate physical fitness regimens for conferring optimal CV health and longevity.
You ask about "excessive running" but the actual article is about "chronic training for and competing in extreme endurance events"
"excessive running" sounds like a lesser amount of exercise than the subject of the study, though you need to quantitatively qualify terms like excessive. A single instance of "> 1 hour of maximal/near maximal running" may not qualify, perhaps doing it twice every day for five years would?
The article says "further investigation [is warranted] to identify at-risk individuals" - unless you are continuously training for and competing in ultra-marathons it may be too early to start worrying.