If all Human beings are 'super complicated' neuro-chemical biological machines (with all possible 'brain' activities governed by the laws of physics and chemistry) how can any 'brain' activities even appear to involve a set of behaviors where one behavior is chosen to be acted on and the other is not?
To see that one behavior is chosen while another is not, by looking at brain activity, you can do something like this: find a set of neurons A that start becoming active before behavior 1 is executed; you find a set of neurons B that start becoming active before behavior 2 is executed; and you find when presenting a choice between behavior 1 and 2 that both sets become active but then resolve with strong activation of one (the one which is chosen).
This sort of thing (anticipatory neuronal activity that is enhanced when an action is selected or suppressed if it is not) is observed fairly often. I don't know whether the exact formulation I gave above has been observed, but there is certainly no reason to think it is impossible, and even the possibility is enough to answer the question.
(Anticipatory neural activity is philosophically and experimentally vexing in its own right, see e.g. here.)