What they probably meant is that at 5°F the pump can supply 100% of the heat required to keep a house (of certain size) warm, with no other form of heating required. At -5 and -10, it can still extract enough heat from the outside to supply 70-80% of what's needed but you will need other means of heating such as resistive electric radiators to complement the heat pump.
Surely not engineering way of thinking but that's a common heat pump metric for ordinary people.
My new car can do 100% driving!