With many things, Musk was right. Both in automotive and in space, people 'insiders' have many believes and many of them turned out to be wrong.
SpaceX and Tesla got where they are by breaking conventional wisdom and if they had not done so, they wouldn't exist any more.
It so happens reinventing mass manufacturing was not as easy as other things. Tesla is still working on it, and they are making improvements that other car companies have not thought about. With each factory and each now Model of car they are interdicting improvements.
The very guy who does the interview with Elon that this information is built on was the former lead engineer at Ford and he believes Tesla is improving faster then anybody. He is incredibly impressed with the engineering and manufacturing at Tesla and how fast it is improving.