It leaves it up to the head to decide whether or not the public interest does outweigh the need for secrecy. It also makes it mandatory for the head to refuse disclosure of any information that relates to national security.
That phrase “relates to” is pretty broad. I don't know if any of you have read in the newspaper today that an interesting book, just published, disclosed for the first time plans the government had in the Cold War period to take dissidents into camps—