Sorry, but this story does not sound plausible at all.
Thinking about this.......all the CC does is take control of the electronic throttle pedal and modulate the throttle to maintain the set speed.
The above is exactly what is done when the CC is turned off, except the throttle is modulated with a foot.
Why should one break the gearbox when the other doesn't?
Until I hear some plausible link I think the SA hasn't got a clue. How do they establish that the box broke in CC mode? Unless the car comes to a grinding halt whilst in CC, how do you link the cause to CC?