I don't think of that as a fact.
Well, according to his tax returns, he did. It’s not impossible, it just takes a lot of savvy and native math smarts, which he obviously had. Most people don’t spend their lives cultivating this.
He had some real estate investments and other such things, but most of his income was from gambling. And, incidentally, I’ve met people even in my small town who have made their living gaming - and I don’t hang out with a particularly racy crowd, believe me. You have to have the discipline (no drinking or participating in the casino life) and the mathematical/statistical ability to predict and interpret.