You are correct, it's not the case. A lot of engines have large enough radiators and enough coolant bypass flow around the thermostat to require some blockage of airflow through the radiator in cold conditions. Below 30-40 degrees I need to restrict the radiator airflow on my tractor. Just the size of the radiator and the bypass flow is enough to overcool the engine in cold temps with full airflow.
Before anyone asks, yes the thermostat is new and operating correctly. I've checked the temp with my IR thermometer. The 45+ year old temp gage is even pretty close.