But as classified information he's legally not allowed to share it publicly as part of testimony or otherwise. We don't actually know what he said in private to correct to those congressmen and women who are authorized to know this information. (i.e. The intelligence oversight committee.)
If the problem was the classified nature of the answer, the proper answer would have been something like "The law prvents me from addressing classified topics in public", not "No... not wittingly". Given that it was congress doing the asking, they would have had any number of ways they could have addressed that concern including giving him immunity or cleared the chamber so he was only addressing congress. Also, as Clapper was given the questions beforehand, he had ample opportunity to bring this up beforehand.
But that's not how this works. By saying that he can't disclose, he's disclosing the existence of the program. It's entirely possible that the program's existence was classified. The NSA's existence was classified for many years and I believe the "Groom Lake Facility" (aka Area 51) might still be. Everyone knows it exists, but the government won't admit it.
He can't address it beforehand if the admission of its existence is classified. By addressing it beforehand and saying "you can't ask me this question" he indicates its existence.
