You're comparing apples and oranges: reported accident != knocking over a traffic cone.
I am asking, if anything, that we take a more critical look at the statement being made every time Chris Urmson opens his mouth. Because when he brags about the safety of his cars, he's ignoring or failing to mention these incidents where humans saved him from having a very, very bad day.
I agree with some of what you have said, but this is the most bizarre statement I have read today.
There are probably hundreds (or more) traffic cones hit for every small child hit by a car, just because there are way more cars per day passing close to traffic cones than to children.
I personally have had my car hit by others 5 times without reporting it to my insurance company (car was parked, hit and run), as the damage was less than my deductible, and I opted to just live with a few dents. Estimating the number of times a car collides with anything per mile driven is very hard.
My intention in comparing a traffic cone to a small child is the actual safety issue. If a SDC could hit a cone, it could also hit a small child. They're small road hazards which are not generally going to be mapped by the car in advance. I don't want to dismiss the fact that the SDC would've hit a road obstacle, just because in that case the obstacle was a cheap plastic object.
I think the point is that if you counted these kinds of incidents in human driver statistics, your miles per accident numbers would come way, way down.