> Over the four years I spent in high school I was assigned as homework every. single. AP Calc I & II problem that had ever been published. Is it really that surprising that our class did well?

AP tests are different than the SAT/ACT. AP tests measure knowledge of something (the degree to which one can study the test independently of the notional subject matter may be debatable, sure), but SAT/ACT are very much proxy IQ tests. They may frequently be used together (but MIT is dropping SAT/ACT, not AP), but are not equivalent to each other.

> If you read voraciously as a child you will score extremely well on the reading section without really trying. You aren't smarter.

There is considerable, though as in most things intelligence-related, not conclusive, evidence that reading, especially early reading, does improve general intelligence.

> I read math textbooks for fun. I was doing college-level math in 9th grade. And surprise to no one I scored almost perfect on the math section. But crucially, this didn't make me smarter than my peers.

How do you know that you didn’t both do this in part because you were already smarter than your peers, and that doing it didn’t make you even smarter than you would otherwise have been?

> Literally anyone who had decided to spend their time at recess reading number theory textbooks would have done just as well.

Even if that was provably true, that doesn’t mean doing it didn’t make you smarter, since if it did it would presumably also have that effect on others who did it.