Presumably the premise is for the blanket to insulate the battery, i.e. the thing that needs to be warm, not the windshield.
But even once the car is stationary, a blanket would help it retain the heat generated while driving, or generated from charging now that it's plugged in.
I wonder why EVs don’t have a built in functionality to keep the batteries warm in cold weather. Presumably that would consume a bit of energy to keep batteries functioning in full capacity.