Cisco wouldn't act without authorisation of starfleet and then continue with yet more breaching of the chain of command by failing to inform star fleet for the months leading up to this episode in question.
Oh, I wasn't suggesting that Sisko acted without authorisation or the knowledge of his superiors. Re-reading my comments, I can see they were ambiguously phrased, and can fully understand why you took that to be my meaning.
What I was trying to say was that the information would only be available to those with an immediate need to know. The ranking officer on the Defiant project would know, and probably be the one responsible for shipping the armour plates to DS9. There may be others. But outside that need to know compartment, the information would not be freely available.