Even if you take Mashable's 16,550 miles driven per year figure, and multiply by 6.4, you still get over 100,000 miles. Though San Franciscans may also drive less miles per year as well, since with traffic, you spend a lot more time on the road going a lot less miles.
But then, we can also go into their statistic basing on a single insurance agency, unreported incidents not factoring in, there's definitely a lot of margin of error here.
I'm glad we're talking about it critically though, and not just glossing over the figures.
One of the other potential points is something Tesla said about Geohot's garage-built car. That it's easy to build a car that performs correctly 99% of the time, but it's exponentially harder to build a car that performs correctly 99.9999% of the time. The number of miles may be less material than even I've focused on here.
Google likes to tout the mileage figure, but it ignores the times that it's human drivers take over. The reality is, driving on straight stretches of road is easy. 99% of driving is easy. Figuring out how to evaluate how safe you are in the other 1% of that time is perhaps an even more challenging problem.