I understood the phrase as emphasizing the "deserving" implication of the word earn. As in, it is not possible for a human to take actions such that the fair reward is a billion. Or in other words, if you got a billion you got more than it's fair. To emphasize: this does not necessarily mean that the way one became a billionaire is nefarious or evil. It just implies unfairness.

I'm not defending that interpretation, mind you, just saying it's a possible read of the phrase.