yes the engine is getting denser air ...but its the fact that it's cold (not hot) that helps things out. The colder air cools your engine as its taken in. Ping is usually caused by one of 2 things...not enough fuel (lean) or your motor is getting to hot. Thats why guys with turbos run intercoolers...not only to get the denser air...but to get the cooler air to keep internal engine temp down.
The cold air/increaed density statement is soooo true. I remember a few summers ago that the FAA wouldn't let jets take off from the Phoenix airport (air temp was over 130 degrees) because the air wasn't dense enough to lift the planes. They would run out of runway before they got up enough speed to lift the planes.