You can rename your Twitter account to any unused handle. All they had to do was delete or rename the account holding @N, then he renamed his @N_IS_STOLEN account to @N. No merging was required.
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.
I had no idea that Twitter could do that(or that, apparently, you can just change your Twitter handle) and so it caused me to read his Twitter[1] as though the thief had decided to mock either twitter or @N_IS_STOLEN by tweeting lots of things about trying to get @N back to its rightful owner.
