Well, chaos theory is explicitly not a denial of determinism, and speaks more to our ability to precisely measure state and accurately model it than anything else. The more precisely you can can measure state, the longer the Lyapunov time, and the longer we can accurately predict the future states of the system.

There are systems today that we can predict fully (i.e. we can predict what will happen faster than it can actually happen) that decades ago would have been unpredictable due to their chaotic nature. There are systems we cannot predict today that we will be able to in the future. Brains very well could be one of them.