"So you could make maybe 80% of the population smarter at that subject than 80% of people with a PhD "
No, you can't. You can, if they reach some minimum standard of intelligence, teach them more about a subject. That does not make them smarter.
Do not confuse the ability to regurgitate facts with intelligence, or even understanding.
Also, do not conflate "with a PhD" with any particular level of learning or intelligence. that's conflating classroom compliance with education, education with understanding, and understanding with intelligence.