Earlier quoted context omitted.
Any adult in America can buy stock and participate. At Robinhood.com, you can buy fractional shares for as little as $1 and no commission on the trade. No, I'm not a robinhood user nor do I own stock in robinhood. It's just an example of how it is never been easier for any American to invest in stocks.
So the logistics of obtaining stock have been solved. That seems like a great and necessary step in the right direction. "84% of US stocks are owned by the richest 10%" sounds like it could turn into a really big problem though. What can we do next? Why isn't everyone participating?
The quote does not state that not everyone is participating (though it's true). Just that the least wealthy 90% own 16% of the (huge) US stock market.