This ignores the potential to build stuff in space, which would create a market for metals, like say aluminum. Sure aluminum is cheap on earth, but the delivery fees to space are $$$$.

This article also seems to have a basic misunderstanding how travel in space works. You spend energy to accelerate/deaccelerate in space, you don't pay per meter like you do in the atmosphere. So once in the asteroid belt you can reach any other point in the asteroid belt with very little fuel, if you are patient.