I drove an early 90's BMW 3 series back in the day which had a speed control system that used a lever on the bottom right side of steering wheel. Once cruise was activated a click forward on the lever accelerated 1mph and a click down the opposite. One day when messing with this feature I noticed that if I held the lever forward my car would just accelerate (at a pretty high rate I might add), as well as making the petal hit the floor without touching it, just like when using regular cruise control and your car is going up a hill etc you can feel the petal move. Yet I was easily able to disengage this sudden acceleration by either letting go of the lever or hitting the break. I eventually starting using this as a lazy driving technique.
My thought is maybe the WOZ is mistaking this "Scary" acceleration as just a feature ( I have not researched this), like the coast function which slows you down. Also the system could be queuing his successive "Nudges" as 10 or so MPH and applying them at once. It does take some time for a car to go from 80 to 81 depending on the car, but having a Prius I do not see it getting from 80 to 81 in an instant...
I understand this could be a scary situation if you have never experienced it but if he is easily able to hit the brake, which would stop acceleration effectively cutting off cruise, maybe he is mistaking this "bug" for a "feature".