I think he's a bit mixed up. That rule of thumb is for when you're pumping from cold area to warm area, which is what you usually do. For instance from outside (10 degrees C air) to inside (21 degrees C air). Then you can see how the performance would drop as the differential changes.

A heat pump should perform really well when the place it's pumping from is high temperature like this. Pumping from 19 degrees C to a house that's 21 degrees C is going to be great.

Obviously things will be much less efficient if the heat pump were being used to warm the Underground from outside air (5 degrees C) but no one's going to do that! The Underground air is going to go into the home (which is at 21 degrees C). The street air isn't in the picture at all.