How to resolve the following seeming contradiction? Mas-Colell, Whinston, Green p. 148 Prop. 5.E.1 says that the efficient aggregate (coordinated) production is just the sum of individually optimal productions. But Adam Smith argued that division of labour allows more efficient production than the sum of individual productions.
Of course Adam Smith's argument is informal, so it is not clear what assumptions differ from Mas-Colell. It could be that Smith envisioned production units with market power, externalities or some other inefficiency and the inefficiency somehow disappears with aggregation.