From Cruise's statements and news reports, it does not sound like the Cruise car was hit by another vehicle. A nearby vehicle flung a pedestrian in front of a Cruise. The Cruise car stopped on top of the pedestrian, then dragged them as it pulled over. Even accounting for post-crash panic, I find it unlikely that a decent human driver wouldn't have realized that the pedestrian ended up underneath their car.
If a pedestrian gets flung in front of your car and then you no longer see them, wouldn't you be inclined to check whether they might have gone under your car? Especially if you're hearing screaming...