Not agreeing with the (somewhat amusing) limited heartbeats theory, but even under that theory, cardio is beneficial since it lowers your resting heart rate.

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.