I don't think they're saying that actual factory workers are unintelligent, but that an AI wouldn't need to simulate a great deal of intelligence in order to perform a lot of the menial tasks humans are made to do. Even many complex jobs or trades are largely task-oriented, demanding skill, but not necessarily great leaps of intuition to perform. Your average human is well well above the necessary intelligence to perform the average job, but we do them because somebody has to (and jobs, but that's a whole other thing).