The term is a metaphor from the difference between intelligence and technical ability. The metaphor in casual speaking ignores the difference.
Rocket science is technically complex and requires mastery in many mechanical fields...more than any type of engineering.
It moreover is a metaphor to state how complex or straight forward a given task is (its not rocket science). It takes years of precise planning, calculating, testing before launching a rocket.
Technical abilities do take some intelligence in order to learn. In rocket science many advanced concepts you need a lot of visual understanding that not many people are able to learn in a given time frame .
Of course that doesn't necessarily mean they are socially intelligent. Many scientists are socially retarded.