The argument does not make this assumption––there is no indication that brain damage is ever (or always) “easily” detectable.

A lack of creativity and innovation is presumed to be a reason that some problems haven’t been solved, but the argument does not presume that a lack of creativity and innovation is the only reason for any unsolved problem.

The argument does not make this inference merely based on the fact that we don’t know what these parts of the brain do. Instead, the inference is based on the observation that significant brain damage does not have discernible adverse effects in many people.

The argument does not make this inference. The argument does not claim that these problems will be solved; the argument only says that the problems will be in our ability to solve.

The author says that 90% of the human brain is unused, then in the conclusion jumps to the claim about “this tremendous source of creativity and innovation.” We have no reason to believe that the unused parts of the brain are a potential source of creativity and innovation.