You are saying that heat pumps get less efficient when deltas are larger, and the parent post says they get more efficient when deltas are larger. In a sense, you're both correct.

There are multiple relevant temperatures for a heat pump, and the pump is more efficient when some of those are higher and some lower. A heat pump has two heat exchangers, one on the inside of the building and one outside. Each of those heat exchangers has two temperatures: the refrigerant loop temperature at that point, and the ambient temperature (air for air source heat pumps, ground for ground source heat pumps). There's also a fifth relevant temperature that has indirect influence: the setpoint (the desired indoor ambient temperature).

Efficiency increases when the temperature delta between the refrigerant and ambient temperatures is higher (both indoor and outdoor). But those temperature deltas vary inversely with the delta between the indoor and outdoor ambient temperatures.

So, in summary:

- Heat pumps get less efficient when the temperature delta between indoor and outdoor temperature is higher.

- They get more efficient when the temperature delta between refrigerant and ambient temperature is higher.

The net effect of this is that heat pumps become less efficient as the temperature becomes hotter outside in the summer and colder outside in the winter.

I see, the previous commenter stated the opposite :). Anyway, both numbers are > 1.