Mostly true, but not completely. After all, they did start up a line in SC.
Boeing can star a line for either brand new components or aircraft models. For realistic purposes they can not move the Washington plant because of the infrastructure already in place. The SC plant builds components and ‘rear ends’ for the tails of the 787. They do some completion work there but only after the aircraft are flying.
OP
(Recovering engineer in the business 15 years until I quit and started my own company)