Scientifically it's incorrect, it's a wives-tale and has no truth to it. Metaphorically it points out the titanic workload of the sub/unconscious compared to the egoic conscious self that's just a chunk of it.
The unconscious - the "90%" humans don't use - is still active and churning, we're just unaware of it sans the "intuitions" "impulses" "hunches" & "instincts" we get from it. The mind is like a horse we ride on, you could say the rider is the "10%", the ego, us. You direct the horse, but the horse does all the work and can collapse under you if it's not paid attention to.
To say we go through life on autopilot is an understatement: most people live life according to the description of the roles they've taken on, they act as a doctor should act, act as a father and husband should act, even when you're hanging out with the boys there's a code of conduct, it's programs we follow to be successful in the world, and we may enjoy them, but they're inauthentic, they're social-constructs. Then throw in the abstract, far off goals that people follow, an imaginary oasis decades into the intangible future. I wouldn't say autopilot, I would say hypnotized, anesthetized.
A being who becomes both the horse & rider? A 100%-er? That's enlightenment, that's what being awake means. To have clear communication & unity with the unconscious, taking in all the information it has to offer would give you suprahuman cognition. You would be getting more "reality" fed to you than most other people, because the unconscious acts as a filter. You would be able to see & interract with systems of reality incomprehensible to the average rider because you are also the horse, you have a connection to it, and that horse has an intimate connection to reality's true face, because it runs on it, and the ego, "you", just ride on it.