No. Even if you work for a Fortune 500 company (which 80% of the workforce doesn't), your CEO likely isn't a billionaire unless they also founded the company. The vast majority of these companies aren't lead by the only 770 billionaires in the US. Even making 15 million a year (typical Fortune 500 CEO total compensation), it would take a long time to hit a billion.
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.
I was assuming the commenter was referring to how billionaires basically control everything through influence, government and regulatory capture, etc. So we're basically their little pawns and compute resources.