My experience is that the ICE will turn on below ~30F even if the HVAC is off and the cabin has been pre-heated on L2 electric. (That is - the car thinks it's below ~30F - parking in a warmer garage will of course, prevent this).
Yeah, we are only just getting to regular temps below 30F so I haven't had a lot of opportunities to see its behavior. I recall that last year I was able to keep the ICE off even below 30, but so far the few times I've tried this year - cabin fully conditioned on L2, HVAC off before starting, the ICE fired right up every time.This is not true in my 2022 - with cabin conditioning and no climate I have driven the van in weather well below 30°F without the ice coming on even when temps were at 0°. It is certainly more likely to engage ice however with any additional needs like climate controls or acceleration etc.