I think it naturally does in that your ability to run on a treadmill for an extended period is quite indicative of your overall health (if the study is correct). I don't think that is really much of a surprise. Health and fitness are pretty tightly coupled.
I am in my early thirties. I do a fair bit of hiking and I can tell you there are lots of 60 years out there that I can't keep up without it being workout. Most of them look great and will tell you they feel great. Is it correlation or causation? I suspect both, the older folks you meet 30+ miles into wilderness on some trail are both the ones healthy enough to get themselves there but one of the reasons for that is very likely the fact they undertake the regular exercise of doing it.
Same thing here, the folks that stay on the treadmill and don't peak out in terms of heart rate are probably pretty healthy. That is going to make them more resilient when it comes to recovery from disease etc. If they are already to sick to do it, they are kinda of by definition already less healthy and are therefore likely to have inferior recoupreative powers when they do get sick.