But that assumes that with the same inputs, the outputs are always the same. As far as I know this hasn't been proven for quantum mechanics. I'm just a layman but in my understanding you can only predict a probability of the result.
That's why with exactly the same inputs, the output can actually differ. Whether or not that's free will is another discussion, but the "we are a very complex yet in theory predicable computer" argument doesn't hold.
If you're willing to accept inputs that are only almost exactly the same, there's already a class of deterministic systems that can provide you with wildly different outputs.
Moreover, in such systems, the output might be impossible to predict based on the input (other than by running the system).
I'm just saying, you don't need to reach for quantum mechanics here, living organisms (and computers) are quite capable of being unpredictable even without it.
That's why with exactly the same inputs, the output can actually differ. Whether or not that's free will is another discussion, but the "we are a very complex yet in theory predicable computer" argument doesn't hold.