The fact is that all heat pumps loose efficiency as it gets colder. I think if you check that they can still heat down to 5 but the efficiency is not there because the heat differential is outside the heating range. If memory serves for an inside there some that go to -10f I remember this number from a training class a year or 2 ago but not the brand or model. We were lucky our heat pump was mounted outside our chimney (with a wood stove insert this kept the bricks warm so our unit would work in the -f range even though it was only rated for 20f) as you can see from the curves when it gets below the teens that max heat starts to drop off, to say it another way you might need 110 f coil temp to get 78f inside the home so when the coil is down to 78f now you’re down to 52 or cold for inside temps. Things that will help keep snow off of and away from the outside unit, if you have high winds a wind break can also help as this will allow the heat radiating from the house to increase the efficiency, hope this helps but depending on the unit it may heat but just not efficiently.
Added to address op comment: I have installed old classic heat pumps with fixed orifices and TXV (temperature controlled throttle valves), there is no real comparison in efficiency of this older single speed system compared to inverter driven or variable speed systems, the old orifice type topped out about 12 or 13 seer where the inverter driven units are in the low to mid 20’s for the most part and improving about every 3 years, the additional improvement is with modern split systems each room is independently heated Or cooled, so you save by not running as hard, minis vary there speed so there is not much starting and stopping and this also saves no ductwork to heat or cool is another plus. a system running at 40% for an hour compared to a old fashioned system starting and stopping at least 3 times, then running at full speed prior to shutdown is less efficient because on startup compressors draw 3-5x there normal running power, in the long run running at a variable speed is easier on the equipment and more energy efficient. We have used this technology in industrial plants for close to 30 years to save $ but it wasn’t until about 10 years ago the smaller inverters actually were robust enough for home use (I used to say they were a waste for anything under 20hp) but a few years Back they started coming out with tiny modules that were lasting as long as the big units, I finally installed one in my home not long ago (not saying the brand but mine and several others now have 10 year warranty’s) where not that long ago it was 1,3 or 5 years on the systems I saw. So it comes down to the type of system and the type of drive controllers and the over all seer rating (the higher the better). With split systems they can be even more energy efficient because you don’t need to heat or cool each room for example, summer during the day keep the kitchen and living room nice and cool, but let the bedrooms be 20 degrees warmer, an night adjust so the kitchen and living rooms are warmer but the bedrooms are just right.
Not cooling the entire house works plus, as the rooms reach the set point the fans slow to a whisper , the compressor slows to just keep up with demand so it is quieter all saving $ and extending system life. This is what I have seen and some of the reasons some can work to lower temps, the type of Freon, the seer of the combined components and how they are set up all make a difference. Hope that helped without being two long winded.