That's like saying "cooling a stationary house is littering". Operating a device for the comfort of the user is precisely why we make things like AC, whether it's in a car, a boat, or a house.
No, there is a huge difference. In a house, the AC is powered efficiently by electric power from the grid or even your solar cells. With the car, you keep a combustion engine , that could output 100kW or more running purely to power a 1kW AC unit. This is one of many advantages of electric or hybrid cars, that you can keep AC running.
In a house, you are cooling the entire volume of space enclosed so that wherever a human happens to go, they are comfortable. It is still littering, regardless of the scale.
Do you use AC? Or any mode of transportation other than walking? Or any convenience that relies on fossil fuels or grid electricity? Do you use any manufactured products? Eat any food that has been shipped a distance farther than your garden?
I assume (perhaps I should not) that by "littering" you are referring to the production of greenhouse gases. If this is so, I'd like to point out that if you are breathing, by your definition, you are littering with every breath.
Yes, I do all of that. I was only continuing the parent's claim that idling a car for AC is littering. I don't actually believe it is - merely stating in that worldview that airconditioning a house is littering too.