Maybe first point to be made - once the engine is at thermostatted temperature, the radiator will not cool the coolant to ambient temperatures, or to anything even approaching ambient.
Second point - if the coolant cools too much, the thermostat will close slightly, restricting the rate at which the cool(er) coolant can enter the block.
Third point - most thermostats have a small bleed hole in the flat part so that water can always circulate, even when the thermostat is closed.
We do get to 0 in Melbourne - not often, but there are the odd early mornings where ice can cover a car left outside overnight.