They ARE NOT "Warm-Blooded"
There is a serious logical error in comparing this fish's heat generation with that of "warm-blooded" creatures.
The observation is that the fish keeps its body temperature five degrees above the water temperature.
However, "warm-blooded" organisms such as mammals or humans do not maintain a certain temperature above that of their surroundings; they very closely maintain a certain ABSOLUTE temperature, regardless of the surrounding temperature.