You are doing the wrong calculation. He isn't claiming to spend 40 hours a week on the prototype. This could be done in spare time, taking advantage of time t

You are doing the wrong calculation. He isn't claiming to spend 40 hours a week on the prototype. This could be done in spare time, taking advantage of time that is probably netting $0/hr, and in many cases probably can be done in less than 120 hours (especially if he locked down the technology stack)