Say the universe is perfectly deterministic. You could look into the future by simulating everything perfectly but why would that prohibit free will? Most people wouldn't argue that an all knowing god would mean free will is impossible, why is this different?
I'd argue that an all-knowing God is something that can't really be reasoned about. It's the introduction of an infinite value into the discussion (or maybe more like a division by zero?). You end up with logically contradictory statements when describing the properties of such a being.
Maybe the general definition of free will most people seem to go with just doesn't deal with weird edge cases because it basically developed to fit reality.