They're claiming that human+Autosteer+TACC is safer than a human. They're not claiming that Autopilot while you use your phone or watch a movie is safer than human driving. And they remind you of this every time you engage the feature.

Consider AEB. AEB is safety improvement on pure human driving. But it'd be hugely foolish to rely 100% on AEB to do your braking for you. It's not designed for that -- it can help you in a lot of situations, but not all of them. The same is true for Autopilot.