First, the study looked at people over the age of 50 who ran 5 or more marathons over a period of three years.
Second, studies on "marathoners" that do not distinguish between fitness levels are completely useless. You just cannot compare a well trained, well conditioned marathoner who can handle the distance with the "experience" marathoner who just trains enough to get across the finish line and is in pretty bad shape by the end of a race.
Third, Duhhhh, multiple marathoning over the age of 50 is going to present some health risks. But, the study only identifies one possible risk, without looking comparing running to all of the benefits.
And, finally, fourth, no one ever said that people do distance running because it is the perfect fountain of youth. I run marathons because I like it. A side effect is that when in racing shape, I weigh about what I did when I graduated from college, 18 years ago. If doing this means that I will have a slightly higher risk of some cardiovascular disease later in life, that is fine with me. My non-running friends who weigh about 80-100 lbs more than they did after college would kill to have that problem in stead of the diabetes, bad knees and back, high blood pressure and all the other problems they have accumulated due to their non-marathoning habit.