You are right about the BB. Everything else has to line up in relation to it.Looks like the BB is the fixed point, and then you can crank on the nuts on the head tube and seat tube jigs to 'twist' it back into shape.
I remember reading about a similar method of straightening a bent car floor in one of my cars DIY repair books. The method is really simple just requires a lot of measuring and brutal force.Mine is a simple version using some of the same principles, just much more troublesome to use. Two afternoons of fooling with it and it's better but not done! lol