You seem to be thinking this is compared to median driver, and not average one: if most accidents are caused by small number of bad drivers, your average will be bad, but even median might still be good.

And this does not even compare the drivers, but simply miles driven.

So I think that 80% of human drivers would likely be safer than Waymo unless they are driving under the influence or extremely tired or distracted.

Note that "13x safer" already implies being in the top 10%, though.