But then can't you trace him based on the hacker news account? Why not just post the text itself on hacker news, is it because HN can take it down while the source should be relatively stable?

How will you trace him based on the HN account? All that's required to sign up is a username/password combination. Looks like reddit is the same. Generate a random password and access them via tor to not get your IP logged and there isn't much to go on.