That's a very interesting post. The problem is that in such extreme cases as North Korea, if the government is setting up this discussion list or has any access

That's a very interesting post. The problem is that in such extreme cases as North Korea, if the government is setting up this discussion list or has any access to it there is no possibility that the conversation was really anonymous. If the government doesn't have access, then mere silent participation without informing would be considered treason. Even if there was anonymous speech going on, Jung merely needs to approach each participant one by one and have them identify which statements were theirs. By a process of elimination, you find who the dissidents are, even if they refuse to talk.