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.