Bluff. If they had it, does anyone think they'd really sit on it and risk it being discovered? No. A nuke, you detonate as soon as its in place.
There's no glory in calling a bluff after the fact.
If they had it, it should have gone off by now.
(Besides the fact that it is precisely what you'd expect to hear from KSM, and could have been thrown out as a Red Herring to take interrogation down a useless trail. And like AJ said, they may have had trouble maintaining such a device. Or maybe they're still trying to figure out how it works.)
In other news, should we make a list of all the people who denigrated the hypothetical capture/killing of OBL as "JUST a symbolic victory," and then look up what they're saying about it now?
