That is not at all what zero sum means. If you're going to use a widely-used term at the core of an argument, you should be sure you're doing so correctly.
So, advertising created value here by convincing people to give their money for shiny stones? Instead of, you know, investing in health, education or a vacation? Overall, was it a net win? On the one hand, one company makes money, on the other hand, many people waste significant resources for a symbolic gesture.
If you take into consideration the money wasted on overpriced stones, it's zero sum. How did people ever get engaged before De Beers?