His history as @N was never lost; you can see all 15000 tweets. He voluntarily (under coercion) renamed @N to @N_IS_STOLEN. All his tweets/history stayed with his account, now named @N_IS_STOLEN. The other guy then registered or renamed an account as @N, as anyone in the world could've done at the time. On Twitter, your handle is just an editable property of your account like your profile picture or your e-mail address.