Here's a different point of view: Carlos Slim has so far been relatively unable to compete in the US. Some people argue that he does well in countries like Mexico because in those places, he can run a monopoly (like his telecom company) or oligopoly with price fixing power, generating large margin at the expense of his customers (who don't have much of a choice in the matter). Thus, his business has plenty of reserves for things like 3 day workweeks - at the expense of the population, in exactly the zero sum game you describe.
But when you look at modern American billionaires like Steve Jobs or Bill Gates, they made their fortune by working intensely hard and smart in very competitive markets, and inventing or making available to the mass-market products that created billions of dollars in value. It can definitely be argued that a single individual can create billions of dollars of new wealth, and not on the "back" of others - there's a lot of very wealthy early Microsoft and Apple employees out there, and billions of dollars of value created for its customers over the last couple decades.