It's written that when someone files a patent and not granted. So when someone files an identical patent, the patent examiner revert the inventor with reasons that why his patent application can't go further ahead, like because of this patent of NSA.

So here, the patent examiner is doing what you are suggesting. He is citing a prior art and while doing so, they have to make the patent available for public.

I thought prior art only worked for published (prior) art? E.g. I can't claim I invented something before you if I never published/released my invention?

Edit: Literally the first two paragraphs of wikipedia: https://en.wikipedia.org/wiki/Prior_art

The best prior art is a previously filed patent application. :)