Earlier quoted context omitted.
Letting the vehicle idle in the driveway while the windshield defrosts will decrease fuel mileage, because the engine runs for longer.
It's not really the fault of "heating" that you're idling your car unnecessarily. I'm definitely not idling my car to defrost my windshield, as I'll most likely pre-heat my car, or if I don't, I'll scrape the frost off.
The person in the root comment proposed that their practice of letting their ICE car warm up to defrost in the winter may lead to decreased gas mileage. This is correct; it will.