No, you are not under a 15 second timer. If you exit right after the interdiction ends, you can leave the game instantly. The 15 second timer only comes when you're "in combat".
Then FD have got it wrong for that specific case, and people using that trick are using an exploit.
Mind you, FD have already said that it was not their intent to allow submission to give you a means of easy escape, and that they intend to fix it.