So it's not just Tesla's use of a heat pump, it's the Octovalve that lets them send heat in so many different directions that makes it more efficient than other EVs. Taking waste heat from the drivetain or battery and pumping it into the cabin is like free energy!
The 50° F threshold you mention is a little misleading when comparing heat pumps and resistive heating. It's important to remember that heat pumps are MORE than 100% efficient because they move heat, not create it. Yes, they start getting less than optimally efficient as the temperature drops, but they're still better than resistive all the way down to 14° F, when the efficiency is down to parity with resistive. At that point you're getting 1:1 (energy in: heat out) the same 100% efficiency resistive would deliver at any temperature range.