The absence of something not happening is proof of nothing.
It's proof that no 9/11-type attack has succeeded since 9/11. To have any validity your post has to assume that the terrorists have not been plotting another massive attack. It is foolish to assume they haven't been. And the Department of Homeland security knows that they have for they have thwarted some.
By his logic, from 93 to 01, there were no terrorist attacks from abroad on the homeland so that means Clinton was very successful in preventing terrorist attacks after the first WTC bombing.