I'd expect a enemy state's ability to get information out of a system would in fact be correlated with insider's desires to chat about what they did at work. Find some bloke who is a bit lonely, a bit talkative and has good security access, send out a hot woman with an unexpected interest in military specifications. I'm no spymaster but that seems like something a spy agency could manage. Might be able to grab a stickynote with a few key passwords on it to sweeten the operation.

> but because people with top secret clearance

The kind of stuff being leaked on War Thunder forums is generally Restricted or Confidential at best

> The forum has it not because it's common knowledge but because people with top secret clearance happen to frequent the forum and like to talk about/boast about the tech they work with.

Not really. The forum has it because the low-level maintenance grunts are on the forums and like talking about the thing they work on, and don't think of the manual as some super-secret state secret because honestly it mostly isn't, and is classified out of habit rather than out of deep thought.

> An enemy state's ability to get an agent deep inside the military, or to entice someone already inside to turn traitor, or to hack through the layers of security to extract data over the network is largely disconnected from an insider's desire to chat about what they did at work.

It's hardly "deep inside", and an enemy state is surely capable of befriending some low-level military personnel, at which point they'll say much the same technically-classified-but-not-super-important things to their in-person friends as they do on internet fora.