>Modern heat pumps easily keep efficiency below freezing. Ours goes to 22F before there’s any efficiency drop. A different model goes below 0F (we just didn’t need it).

No they don't. Their efficiency is linearly correlated with the ambient temperature. They don't "keep efficiency" nor is there a concept of "efficiency drop". It's just a straight line down.

https://docs.energytransitionmodel.com/main/heat-pumps/

You’re right. My wording was imprecise. I should have said the efficiency drop off is such that for practical purposes it doesn’t matter - until you hit a crossing point. You won’t need supplemental heat until then. Newer systems have pushed those points lower.