This is a good example of Clinton's pathological orientation to the truth, i.e., he tells a lie when the truth would be better.
A better reply, one that conservatives and liberals alike could accept would be something to the effect that if he knew 911 was going to happpen, of course he would have used ever resource to kill him. Nobody can see the future though, there's no crystal ball in the White House.
You have to understand his mental pathologies in order to know that truth-vs-lie isn't relevant in BillyWorld -- just the desperate, manic need to make himself look good and secure that elusive legacy.
If the truth serves that purpose better, then he'll go with it; otherwise, he'll blurt out whatever lie randomly pops into his fat head, regardless of how implausible it sounds. Hell, the more outlandish, the more he *wants* us to doubt him so that he can play victim of the VRWC.
The best thing to do is to mostly ignore him, although occasionally laughing at him is OK.