It's definitely in line with general thermodynamics. One would expect lower entering water temperature to be more efficient (especially with the condenser being counterflow). I'm just not sure if there's something specific about the design of real world heat pumps that makes it so that higher delta T across the pump is problematic. I thought I would have come across some hydronic professional explaining this a bit more in depth while focusing on each variable, but so far I have not.

Maybe it never comes up in practice because heat pump systems inevitably need some kind of buffer tank. If you're designing a system from scratch then you design for lower delta-T in the emitters to keep the max water temperature down. And if you're using existing emitters then you just live with the inefficiency due to higher max water temp, but still keep the flow from the heat pump to buffer tank high regardless (to keep the max water temp from being even higher).