Hi all, interesting discussion.
This is just anecdotal evidence, but last time I checked, consumer literature on heat pumps, especially in the Netherlands, tended to be focussed on ground-based heat pumps. To make matters worse, such systems were also referred to as geo-energy systems.
Checking on the site of milieu centraal it seems that currently the focus is on hybrid gas-fired boiler / air-based heat pump combinations, so I’m not sure I can back up that statement.
At any rate, for ground-based heat pumps, the temperature dependence of COP is much less relevant because ground-based heat pumps operate with much smaller external temperature changes. And it could be that the authors of studies you’ve mentioned assumed this implicitly.
Regards,
Bart