So the sicker you are the less in shape you are in? Seems obvious to me.
There are historically lots of things that "seemed obvious" but couldn't hold up under research, which is why we do research. It confirms what you think or maybe it forces you to question what you think. Either way, it is good.
Case and point, there is a rising opinion lately which Trump has promoted that your heart only beats a set, finite number, of beats in a lifetime. Therefore, according to this opposing hypothesis, it makes sense to not perform intense cardiovascular activity throughout your life, because it accelerates your heart hitting that finite limit that it is capable of. This opinion has gained traction in recent years and to some people it "seems obvious" that a faster heartrate might wear it out more quickly, leading to premature death. So to those people, research like this defeats their assumptions and helps them feel more confident about performing exercise because it is research-backed that Cardiovascular Fitness prolongs life.
That's why research is important. Obvious shouldn't stand for anything.
e.g.
my HR is 50 thanks to running an hour a day, my HR during those runs are between 120 to 150, that gives 50x60x23+150x60 == 78000 beats a day
a non runner might've a HR of 60 (being very generous, most folks are well above 70), that gives 60x60x24 == 86400 beats
Again, the limited heartbeats theory sounds like an oversimplification. But the point of rigorous training (cardio and strength) is that your body adapts to the taxing efforts during training and doesn't need to work nearly as hard for your day to day.
Sources? The claim I most recently heard is that 60-80 bpm is within normal range (which itself surprised me, because I've usually been told a target of 60 bpm resting heartrate, leaving aside the 40 bpm achievable by some athletes).
The study itself can’t address the direction of causality because it is observational.