The Prius has a "power" button, which is enabled by inserting the key fob in a slot below the button. I don't know what happens if you press the power button while on the road or remove the key fob, but I would expect it to turn off the ignition.
In any event, the Prius also has a transmission that can be shifted into neutral, or at least into "park". I doubt that those changes would also be ignored by the throttle.
This is simply another case where the driver panic'ed.
Dunno...the Prius has a strange transmission.
For example:
“Reverse gear: There is no reverse gear as in a conventional gearbox: the computer feeds negative voltage to MG2, applying negative torque to the wheels. Early models did not supply enough torque for some situations: there have been reports of early Prius owners not being able to back the car up steep hills in San Francisco. The problem has been fixed in recent models. If the battery is low, the system can simultaneously run the engine and draw power from MG1, although this will reduce available reverse torque at the wheels.”
http://en.wikipedia.org/wiki/Hybrid_Synergy_Drive
Very weird.