I don’t know who Graham is talking about—“politicians”?—but this is a very famous claim, or corollary, from Marx: capitalists exploit workers. Why? Because of the labor theory of value. (1) All value is created from labor. (2) Capitalists employ workers in order to extract surplus value from them (what’s the point of employing them if you don’t get more out of your “investment” than you pay it?). (3) Ergo capitalists exploit workers.

Of course millionaires and billionaires can still _work_ themselves in the sense given above, i.e. in a way that creates value. And one could argue that one can be a self-made (no exploitation of others) millionaire. But it becomes farcical to claim that a person can become a billionaire without exploiting people. Any person on Earth would need much more than one lifetime to create a Billion USD worth of value _through their own labor alone_.