A reply does no such thing. They can say that your request is denied without even looking to see if the information exists first.

They're not saying, "It's there, we're just not going to admit it." They're saying "We're not even going to begin looking for it."

I think gp was saying: to reply to your letter is to confirm the existence of the body, the NSA itself, who received and replied to your request.

Now we know they exist!!!

We always knew NSA existed. Some of the other guys, however...

I suppose at least this means they have yet to pull "the greatest trick the Devil ever pulled" ("convincing the world he doesn't exist")