I understood, perhaps incorrectly, that the ground below the frost line is stable year round and if air source heat pumps work more efficiently down to -10C, we know the water isn’t that cold (otherwise it would be ice), so wondering if the efficiency threshold is the same for ground source heat pumps?

That's why the ground ones are dug vertically sometimes, they can get to stable temperatures.

However, a well may be "just poking" into that territory so you get the right temperature, but not enough surface area to do much heating/cooling.