Being transparent about such incidents is also what stops them from potentially becoming a business/industry-killing failures. They're doing the right thing here, but they also surely realize how much worse it would be if they tried to deny or downplay it.
Was it unpredictable? They drove past a blind corner (parked SUV) in a school zone. I'm constantly slowing down in these situations as I expect someone might run out at any second. Waymo seemed to default to the view that if it can't see anyone then nobody is there.