Bunch of BS. There is not one secret, one source, one method from 1963 that needs to remain secret. They are protecting the reputation of the CIA, and the fact that it was a coup de tat.
They afraid someone will find out about the new SR-71 project or the Atlas missile? Or that they are working on a computer that can be contained in a single 1000 Sq ft facility? The name of our man in East Berlin?
Its a coverup and nothing else.
Yes.
and the fact that it was a coup d'etat.
No, they have been protecting the information that the CIA knew enough about Oswald's plans to have warned that he was about to do something.