Is Bill Gates wrong about supply demand curves?
No, Bill Gates is correct here. The simple econ 101 cross model is based on a separation of the supply and demand side. Firms are price takers, they assume they can sell as much as they want at the prevailing price and their output decisions have no impact on the price. If firms are not price takers, they have to explicitly take into account how the demand side reacts since this determines the impact on prices. Now with huge returns to scale, price taking is usually not a sensible assumption and therefore the econ 101 cross an inappropriate model.