This wording is inaccurate. It isn't about "modern" heat pumps, its about if another $2-3,000 of electronics and clever injection systems are worth letting the refrigeration cycle work at the extremes. For a lot of the U.S. there is just no need to have a system that is a lot less repairable just to get that efficiency below 50F. The complexity differences between a variable speed system that can work over a 100F degree range and a standard single stage system with just a reversing valve are pretty big. The cost is basically never regained in savings in large parts of the southern U.S. so you just don't see them unless they are overseas models made with cheap labor; meant to be thrown away when they break.

Lots of cold weather heat pumps can be had for <$3k total, it’s not $2-3k of additional electronics.

And beyond cost savings on power (especially not needing to burn $30+ of electricity per day because temps dipped below freezing and resistive heat was needed), they just work a whole lot better, and the variable speed fans make them a whole lot quieter, inside and out.

They’re also simple to install yourself, so you don’t need to pay contractors the absurd fees they’re demanding currently.