At this scale it can help to think in terms of mass rather than energy. The most energy the sun could ever emit over its lifetime is if it was completely converted into energy. However, this merger emitted 15 times the mass of the sun as energy. I don't have all the numbers on tap for supernovas but given that the sun won't convert all its mass to energy, it's not hard for a supernova to convert more mass in its explosion into energy than the sun ever will.