You're missing the point. Usually these games have a number of currencies and no fixed exchange rate between them. Your sunglass analogy uses a single currency

You're missing the point. Usually these games have a number of currencies and no fixed exchange rate between them. Your sunglass analogy uses a single currency. The article also talks about the lack of profit in using the currency. You buy gems or diamonds, but spending them doesn't help game progress, but just provides visual flair.