That is exactly what they're trying to find out. From the article:

"My question has always been, how much exercise do you need to do to injure the heart?" Levine says. "Since Ben was planning to swim across the Pacific Ocean, we thought, hey, this might be a good opportunity."

It could go on to show that extreme (exercising at low intensity for 8 hours a day for 5-6 months) isn't "dangerous for your health".

"It could go on to show that extreme (exercising at low intensity for 8 hours a day for 5-6 months)"

That doesn't sound extreme by any definition. It sounds really hard and is a lot of output over time, but it is not at all what I would characterize as "extreme".

If I were trying to discover what "extreme" exercise did to ones heart (or body, whatever) I would simulate a life or death scare that would enlist adrenaline and then present an insurmountable obstacle (basically a steep hill, or staircase that doesn't end) and have them run to muscle failure.

That sounds like extreme physical output and that would be an interesting scenario to test heart health in.d

8 hours of exercise each day is an extreme. A sudden start (like you describe) is also an extreme. The latter is easy to measure and test in a lab (sort of), the former isn't really. It'd be hard to find the funding to pay people to perform that level of physical activity day after day (constant motion, not the start on stop of most pro athlete training schedules), for months at a time.

Surely you can easily find people in poor agricultural countries that do manual labour in fields for many hours everyday. How about women in paddy fields, bent over tending the rice plants for several hours each day?

I wonder if going through special forces selection processes, which often include extreme endurance and nasty resistance to interrogation training, have any long term health impact?

Ah OK, my bad. In the end, it all comes down to "Damn, the human body is a fantastic machine".