..., defied expectations that it would weaken over cool Atlantic waters ...
Note that it
defied expectations, which isn't at all the same as defying science.
I have always heard that as low as 70 degrees, the effect is to sap energy (heat) from the storm, not add to it.
As far as I know, a water temperature of 70 would not sap energy, and could add energy. What saps energy is the raining that releases some of the energy from the storm. At 70, this energy loss would not be fully replaced by the moisture due to the water.
Presumably in this case there was less raining than usual, so the storm could retain energy for a longer period of time.