For many people the physical description of the brain does actually have free will.
The point is to change people's intuition about free will, based on all this new information we've discovered in the past century.
"external to our physical reality" There is no need for that.
Is the world inside of "No mans sky" game "external" or "internal" to our physical reality? Soul is something that is run on the hardware of the brain, and thus can have different properties than the physical properties of neurons. It's virtual. No less real for that though.