In the high frequancey the impedance of the coupling capacitor used in the design became very small so the the power plane and the ground plane during the fast edges became short circuits so any plane will act as return path.
To put in a layman's language, power plane provides for a dedicated layer which is solely meant for a robust, un-interrupted and low impedance power distribution system on the Pcb, whereas ground plane as the name suggests, provide a good reference to the signals on the pcb and a low impedance path to the return currents.