Driving your car is more risky than the odds the A/C happening to fail and the car abruptly overheating in a minute. A serious car accident is not so likely, but why take any chances?
I don't think that is what they meant. It read to me that they meant, a personal emergency could happen to _them_ while in the store (heart attack, fall and hit head, robbery, etc..). What would have been a safe 5-minute trip could turn into a multi-hour ordeal, and nobody knows to go rescue the kid stuck in the (now lethally) hot car.
Probability may be low, but it is an increased risk that can be mitigated to a certain extent by just not getting in the habit of leaving your kids in the car when running even short errands.
