I guess you are talking about the model S?
"For the demand they have now it seems the batteries could be built by a contractor"
They are not building this battery plant for the Model S. It is for the Model X and Model 3. The model X is a crossover vehicle and Model 3 is a low cost electric "for the masses". They are talking 500,000 cars per year.
As for a contractor, Panasonic is taking an equity position in this plant and it is entirely possible they will be manufacturing batteries for other electric car manufacturers.
He is selling cars but he is really selling batteries because that is the most expensive component in the car.
Remember when GE made appliances as a way to sell electric motors and controls. Eventually, electric motors became cheap and GE got out of that business, but they still make electric motors for train engines and turbines for windmills.
My guess is that he’s thinking beyond cars, because the demand for electric cars are just not high enough. There are probably more demand for things like rechargeable storage for solar power units.