Interesting, sounds like a water to water heat pump (takes heat from sewage and pumps it into district heating). Just to dig into your example a bit: It kind of depends on what the return temperature is of the 90C heating fluid is. Based on the refrigerant and COP of 3 (18/6), its probably around 50C. If they were able to find a use for the process fluid so that it returned at 20C, they would probably have a COP closer to 5. You can still build 'relatively' efficient heat pumps for hydronic heating, but they would be significantly more efficient if we did a better job dumping all of the heat from the water (which would require a larger, more effective heat exchanger).
Edit: I guess what I'm trying to say is that most of the radiator technology we use (like the in room part of the heating system) was designed with combustion-based heating in mind. Combustion based heating sort-of works just as well if the return water is 50C or 20C, but you would get a substantial boost in heat pump hydronic heating performance if you were to redesign heat exchangers to get as much temperature out of the fluid as possible. This is an optimization problem: as you increase the size of the heat exchanger, it gets more expense, but improves performance. There is an optimum in there somewhere...
In general, the evolution in district heating networks is towards lower temperatures, allowing better efficiencies with things like heat pumps. As you say, integrating heat pumps into existing district heating networks designed for combustion based heating is suboptimal, but redoing the network for lower temperature operation is incredibly expensive as well.
Currently, in the case referenced above, during the winter the heat pumps are AFAIU the "first leg" when the cold water returns to the plants; the output of the heat pumps is then routed via the fossil fueled CHP plants for topping up (depending on how cold it is, it's heated up to IIRC ~120C) before it's sent out in the network again. The challenge is that due to climate change, the fossil fuel plants need to be replaced with something else (no, biofuels are not an environmentally sensible solution). What that something else is, is not entirely clear. Moar heat pumps + wind + heat storage? Or geothermal? Or nuclear?
Edit: I guess what I'm trying to say is that most of the radiator technology we use (like the in room part of the heating system) was designed with combustion-based heating in mind. Combustion based heating sort-of works just as well if the return water is 50C or 20C, but you would get a substantial boost in heat pump hydronic heating performance if you were to redesign heat exchangers to get as much temperature out of the fluid as possible. This is an optimization problem: as you increase the size of the heat exchanger, it gets more expense, but improves performance. There is an optimum in there somewhere...