Musk probably isn't screwing around. I've been reading comments but no one has mentioned it: Who's to say he doesn't have access to one of the grounded 787s? He could have flown his MIT friend over to take a look with him and torn one of the batteries apart. He's the type of guy who could personally know the owner of a 787. Without schematics, he can tell a lot from the work Tesla has done. He could probably even see a near exact configuration they already tried.