Yeah sorry, the WSJ is not available online for free. I don't see the information you
ask about in the article. The article says that blood is more "clottable" for some people during long distance runs, and also mentions "water intoxication with acute brain swelling" something that has to do with drinking too much water during a marathon, which I had never heard of. The research isn't accepted by everyone, but of course we have all heard of people dropping dead during or after intense exercise. The point as I see it isn't whether this is due to previous health condition but if strenuous exercise exacerbates that condition, to the point that you were better off doing moderate exercise instead of running the marathon or doing the century.