To address the orignial question, the Prius is a throttle by wire vehicle, out of efficiency, but also out of necessity. So, when you are using the throttle, you are really just communicating a throttle position to the computers and they translate that into speed and acceleration, or decleration. Since the whole throttle system is really nothing more than a ground speed selector, the cruise control system works in the same manner. If you set your cruise at 60, the vehicle will command the hybrid system to maintain a highway speed of 60. And it does a very nice job of this on flat and up hills.
Downhills,coast regen does kick in, and can have the effect of maintaining your setpoint, but that is really a function of how steep the downhill grade you are driving on is. The Prius will pick up speed on steep downhill streches, and the use of brake or B mode is necessary. So, the cruise contorl is really just another function of the onboard computers.