I don't see how this is a worst threat than the current situation provided there plenty suicide bombers available.
Because brainwashing a suicide bomber takes time and effort and he can only be used once. And during that time and effort, there are lots of fail points and exposure to the authorities finding out about the suicide bomber. An individual hacking an AV to direct an attack doesn't require very much in the way of infrastructure or organization or time or effort beyond what is already in place. And the exploit (and it can hardly even be considered an exploit, since it's basically using an autonomous vehicle for what it was meant to do, which is go from location A to B). A suicide bomber requires an organization. The attack described in the article does not.