Hmm, I disagree. A purely resistive heater will have an efficiency of about 1 all the time; a heat pump may have a coefficient of performance of say 5 in moderate temperature gradients, reducing as the temperature delta rises. However, the COP can't ever get lower than 1 in theory, because any energy used by the compressor will still eventually be delivered as heat.
Certainly there's no physical reason why a heat pump can't work below 273K, which is an entirely arbitrary value we just happen to be familiar with; after all, when you make liquid nitrogen or any other cryogenic substance, you're using a heat pump to pump heat out, and you're able to reach very low temperatures.
The theoretical efficiency of a heat pump is related to the temperature
difference you're 'pumping against', and the absolute temperature (like Carnot efficiency for heat engines, it's largely equivalent).
A heat pump may become less
effective, though, outside of its design temperature range; depending on the working fluid, complications like ice formation (i.e. insulation) on the heat source, and so forth. It may be that the output (and input) goes to near zero when the working fluid no longer evaporates or whatever, but that's not the same as
efficiency.
I am kinda disappointed in the way the Tesla manages heat, but I'll just have to accept that the various bits I'd have done differently have indeed been carefully considered and it just wasn't worth doing the way I'd have done. Aside from the lack of ability to heat the cabin with a heat pump, another example is the lack of ability to use waste drive-train heat in the cabin (
https://teslamotorsclub.com/tmc/posts/3897624/); I might guess that under conditions where you would want cabin heating, there's no (or very rarely) "spare" drive-train heat available (presumably during Supercharging we don't care about efficiency and are happy to use the resistive heater...).
Anyway... I'm just going to have to get over worrying about every last bit of efficiency, especially in relation to Tesla's notoriously bad vampire drain.