He barely escapes by using the word "proven." Otherwise there are several cases of people executed who we pretty much know were innocent. Aside from that, there are many people who have been released from death row when proven innocent., but this can be attributed to the justice system working.
However, many people have been released from death row only because of DNA testing. If DNA testing didn't exist, those people would be dead. That doesn't get the justice system off the hook, it only means that, luckily for those people, technology made up for its inadequacies.
Aside from the gross inadequacies of our justice system, I fully support the concept of a death penalty.