Not geothermal in the sense you might be thinking (tapping into geysers); rather, it is taking advantage of underground temperature remaining constant around 55F, essentially using the soil underground as a giant and consistent heat sink / heat source.

My first thought was whether heat demands would overwhelm the capacity of the ground, but I read that soil has a thermal mass about 1/5th of water so a rule of thumb might be that you don’t have to worry unless more than about 15% of the total volume underground is replaced with heat pipes, which it is definitely not (first concrete numbers I could find suggested 10cm boreholes spaced 5 meters apart, which is less than 1% of ground volume).