It's kind of strange that it just fried the power supply but that's good news. There's no reason the power supply would act as a surge protector. The way a surge protector works is to use a choke coil in the circuit to hold back the surge in voltage. Anything over 120 volts would be routed back to the surge protector's ground wire and then out of the house. To my knowledge, the Xbox power supply doesn't have any of those components but for whatever reason the surge still didn't make it to your Xbox. Strange.