What evidence is there for another NSA hacker? Proof by narrative. The narrative says that the NSA is impenetrable, except for inside threats. Therefore it was an inside threat.

Proof by narrative is the slippery slope whereby the state becomes a religion.

It's borrowed from religion and used in the modern state. In the old testament, whenever the chosen people were defeated, the only reason could be that someone was making sacrifices to the wrong god somewhere. The proof was that god would have been with them on the battlefield otherwise.

It's why, in Stalinist show trials, political enemies are made to confess for crimes of sabotaging production, because the narrative says that the production process is perfect in the Soviet Union and the only way that quotas would fail to be met is because of reactionary saboteurs. Fallacious or not, each additional conviction reinforces the state's narrative.