This is the best summary I could tease out of the follow up paper:
Although, as we show in , determinism may formally be shown to be consistent, there is no longer any evidence that supports it, in view of the fact that classical physics has been superseded by quantum mechanics, a non-deterministic theory. The import of the free will theorem is that it is not only current quantum theory, but the world itself that is non-deterministic, so that no future theory can return us to a clockwork universe.
See it? At a certain level, future events are inherently unpredictable. These small uncertainties bubble all the way up to our level. So, while we can predict with confidence that the sun will rise tomorrow, certain other smaller events are inherently unpredictable. That's a a circular way of saying that subatomic particles and big things like people have free will, because at least some of their actions cannot be determined by past events and circumstances.
They do this with a proof that first assumes such a model of events exists, and then go on to prove such a model is mathematically impossible. There are no hidden variables or forces, because quantum mechanics won't allow any. The world is non-deterministic, and it's no longer possible to prove that it is deterministic.