These kinds of things are always set up as a hybrid system, where they have another heating element they can use when the outside temperature is too cold. My house has a heat pump system with gas furnace backup. The cutoff point is set I think at about 39 degrees.
I don’t believe the Model Y has that, or at least no information has come out on that. Elon mentioned it had a “special mode” that allowed it to “short” out and run in a less efficient mode to generate heat in high demand cases (heat through some other method compared to how a heat pump normally works) but didn’t go into detail. I can’t imagine it works as well as a dedicated 3kW or 5kW resistance system... but maybe? Just a little concerned since the car shipped after winter was larger over. The heat pump in the house is largely useless in periods of deep cold in the winter (we occasionally get 10 to 25 degree days and nights maybe 20 or 30 times a winter). In fact, the heat pump alone isn’t able to keep up with the demand even when at the set point without occasionally kicking in resistance heating. (Very old house, poor insulation. But then again, a car has pool insulation & a lot of glass radiating away that heat!)