Your refrigerator is a heat pump, rejecting heat from the back and cooling the space in your fridge. A domestic air source heat pump is the same principle except the heat is carried away by water to your radiators and your heat pump is cooling the atmosphere.
In the case of your refrigerator it becomes less efficient in a hot environment. We all know not to put a fridge near a radiator.
In the case of a heat pump with Hydromx in the radiators, more heat energy is carried away from the heat pump and expelled through the radiators making it more efficient. No different to putting your fridge in a colder room to make it more efficient. The compressor in the refrigeration cycle does not need to do so much work therefore using less energy. The CoP ( Coefficient of Performance) is increased. In a recent trial in Korea, Samsung demonstrated a 22% uplift in the overall performance of a heat pump CoP.