Hayden himself is not that bright. He at least lacks emotional intelligence. He once went on a long rant during his live video that gay people are gay by choice and nothing anyone says will change his mind.
For a lot of them, bodybuilding is easy for them because their bodies genetically grow faster. And on top of that, many of these guys use steroids.
I am not accusing Hayden of using steroids because I do not know. And I don't hold it against them. But it most certainly is not a "science" in the way you are describing. I once dated a really built guy who spent 4 hours a day working out. Dumb as a brick. He knew it too and admitted he. He even said that sometimes he wished he had more to offer than his body because he knew he was dumb. He was trying to be a bartender so I helped him study. He really struggled through it and had to take the class four times longer than everyone else. Luckily, his teachers and classmates (in every cycle) were patient with him because he's hot.
Anyway, bodybuilding has zip to do with IQ. I met all kinds of guys with various genetics that help them make muscle. This one guy had a brother who is extremely built bc he builds muscle easy. But for him (the guy I was dating), he was only able to build his legs easily. He only worked out his legs for an hour once a week and his legs and ass were huge (perfect to me since I'm a top). He spent 5 days on his upper body and always complained that he wouldn't grow like his brother.
I dated another guy who was genetically built and ripped and all he ate was junk food and cereal. He never worked out.
I dated a built Brazilian guy who was naturally built as well. He worked out sometimes for fun doing crossfit. He never worked on his legs and they were massive and he thanked his father for that.
There is a stripper I met in person, his legs and ass are huge (his ass is the greatest ass in the world). And he also said he never works his legs, just his upper body. He said his whole family has huge legs.