This seems like it would be rather easy for the government to narrow down. Check the logs of who applied for an NSA job on or around the date the screenshot was posted and cross reference any that are/were located in South Korea. I would think that would produce a rather short list that a bit more investigation would crack.
The guy seems arrogant, and arrogant = sloppy. He'll get caught.
He knows he's about to get caught, reason why he hurried to knock NSA's door. They might let him in after all.
But probably after they arrested him, to help with negotiations.
And to pop that bubble of false confidence.
The way he acted, would be a very red flag for me, if I were to hire him. Maybe skillfull, but careless. And that is not acceptable in that line of work. (Neither it is in the military)