Except that just a few vehicles out of the millions that are on the road. That's an insufficiently large sample size to say how automated cars from different manufactures with different levels of maintenance under varying road contritions will interact. You can't assume competency from the limited, though still impressive, testing Google has done.
If anything you are demonstrating the author's point, assuming that what Google has accomplished will be true of all driverless cars. Each of Google's automated cars is effectively a student driver with Google's engineers, technicians, and drivers shepherding the vehicles through the hazards of everyday driving. How will that record hold when one of those cars are twelve years old and hasn't had a tuneup in three?