The terrorists (a very proper term for them) may not have known the code book had been lost. Some people are hesitant to admit they screwed up so badly, especially when you're dealing with people who have no hesitation about killing others.

If the terrorists knew the code book was lost, they might not have known it was found by the police. The true value of the code book is the ability to read the terrorists' mail. Reading their mail helps us learn what their plans are. If they change their plans, then we're back at square one.

If the terrorists knew the code book had been found by the police, that would be a different situation. Still, the best answer is to never release this kind of information so the terrorists are kept guessing. IMO, if this story is true, whomever released it did a terrible amount of damage to our intelligence efforts.