Technically if you own any stock you're benefiting. If we go by that metric that's almost 50% of people. However, the better question is how the ownership is distributed. One source says the top 1% own 56% of stocks, which paints a less rosy picture that what you're suggesting (that the gains are distributed among the top 20%).
https://financialpost.com/investing/how-americas-1-came-to-d...
Rich is having a high networth. It's a lot more dependent on what social class in which you were raised (and therefore what networks you can access, and what role models you have) than your IQ.*
My IQ puts me easily in the 95th precentile of tested individuals, but I grew up poor to two alcoholic high-school dropouts.
On the other hand, I have a friend who grew up with CEO father who sits on boards of major public companies. This friend spent college (at a lower-to-mid tier school) as a D1 athlete pounding beers while dealing and using massive amounts of cocaine. This friend will tell you I'm the smartest person he's ever met. He's said it to multiple of his contacts while in my presence.
He pulls in hundreds of thousands of dollars a month due to his daddy's connections, while I scratched and clawed my way to rise above my station and get hired by a FAANG.
*This is completely ignoring that IQ is based off an extremely biased, standardized test and a horrible proxy for intelligence.
Of course, there are a few outlier founders, CEOs, and stock traders that are benefiting much more then average. What separates these outliers from others isn’t IQ, because there are plenty of people with very high IQs that are doing about average economically.