If the thermostat is closed, what difference does airflow through the radiator make? If the engine isn't up to thermostat opening temp, it shouldn't be circulating coolant through the radiator anyways, so changing airflow through through the radiator shouldn't affect coolant temps.

Airflow through the radiator actually does two separate things. Airflow through the radiator changes the coolant temp in the radiator, lowering (or raising) it depending on ambient air temp. That air then exits the engine compartment, usually out the bottom, and removes the heat trapped in the engine compartment that radiated from the engine, headers, etc. Many times blocking the airflow through the radiator doesn't change the cooling systems heat loss as much as it changes the heat loss through the loss of the "radiant" heated air in the engine compartment.

Many times a chin spoiler will help a car cool down, and people assume it's from additional airflow directed up into the radiator - it's actually from the spoiler lowering the air pressure under the car, effectively sucking the air out the bottom, and the engine compartment air being able to exit easier.

Also, many semi engines use air pressure controlled fan clutches. In cold weather, many times while idling overnight they won't heat to thermostat opening temp, even though the fan isn't turning. The engine at idle doesn't generate enough heat to overcome the heat loss to the outside air, and/or the cabs heater cores.