Properly mixed engine coolant will not freeze. The block heater preheats the engine to ensure combustion by thinning the lube on the cylinder walls allowing the engine to rotate faster during the starting process. This warming and faster rotation improves compression and warmth of the fuel air mixture which enhances combustion, thus all but guaranteeing a start.
Newer vehicles with let’s say 0W5 motor oils do not require block heaters as the oil pours like water in the winter anyway.
I spent almost eight years stationed in North Dakota and properly mixed coolant froze all the time. Yes, the heaters kept the oil warmer but engine blocks froze all the time up there and it was not due to improperly mixed coolant.