It still not even close to the majority of Americans no matter your definition. The vast majority of the stock market isn't held by billionaires either. The total net worth of all US billionaires is ~5 trillion and the stock market is worth ~40 trillion.
You COULD say that, if you define "working for" in a particular way.
But if define it that way, then "working for" becomes a phrase that has no real use. "X is working for Y" just means "Y has a lot of influence in the world" and says nothing about the relationship between X and Y. So I don't recommend using that definition.
Even if you start counting anyone that works for a company even partially owned by a billionaire, about half of us employees work for small businesses: https://www.forbes.com/advisor/business/small-business-stati...