I don't know about this one in particular, but I hike. Endurance, but not at the level they are talking about. I had a cardiac stress echo last year and I've had an EKG since--both show changes that don't seem to concern the docs. Dr. Google comes back with this might be a risk...or this might be normal for an athlete.

Several of the people I exercise with have ended up having serious cardiac issues that seem to be related to endurance exercise.

I always try to be careful to take off enough rest days myself, but maybe that doesn't even matter and it comes from cumulative adaptation?