The lack of a clutch pedal probably has something to do with it, but not exactly in the way you're proposing. In the Audi case, investigators concluded that pedal placement had something to do with the incidents. Here were the relevant facts back then:
1) For virtually every incident, the Audi was the driver's first non-American car.
2) Distance between the brake and accelerator pedals in Audi 5000's was less than in most American cars (probably because they used the same pedal cluster as manual-transmission models)
3) Pedal height was different from what drivers of American cars would expect. Audi placed the brake and accelerator pedals at roughly the same height to facilitate
heel-and-toe downshifting. Most American cars at the time had the brake pedal substantially higher than the accelerator.
All these factors taken together meant that some drivers would get confused about which pedal they were depressing. The greater prevalence of manual transmissions in the rest of the world would help to explain the lack of such incidents outside of the United States.