Thinking there is a difference between believing that a computer can simulate a brain and believing that a windmill can do so doesn't depend at all on what you think a brain is, or on what a brain actually is.

The case that was made by the parent is that since a computer can simulate a windmill, there is a distinct difference in the relative likelihood of the beliefs, not that either was actually true. Depending on the idea that the brain was a computer to prove that it could be simulated by a computer would be pretty much assuming the conclusion.