The sun does influence our moods for example. Check how in Scandinavia there are greater occurrences of depression due to lack of vitamin D. the food we eat also affects us. the people we talk, childhood trauma. Everything really at least on our closed environment does affect us.
I see this as a flaw in his reasoning: even when my brain and knowledge is the product of past events that were not under my control, that doesn't imply I cannot use my brain,
Nobody said that you can't use your brain. We are simply saying that our thoughts and actions are not really ours.
How a machine has come to be does not necessarily predetermines its use and outputs
nobody said that it predetermines its "exact" output. We are simply saying that the output is not of the computer but of the user. Humans input info to each other so nobody turns out to be a "pure" author..hence no free will.
You only need to be a partial author of your outputs to have shown free will. A machine that can create its own algorithms will be influenced by many inputs from the past, but can still add a layer of independent thinking and so decide what to do or say, no matter what influenced this machine previously. Hence free will *-).
This is the question. How do you know? How do you differentiate?