You are correct that economics is not a zero-sum game. Investors pay money to buy stock in a company, and that company tries to create value for its shareholders. If they succeed, then it's a positive-sum game for the shareholders, employees, and customers. When enough companies create value, then the economy grows, and the economy is a positive-sum game.
However, this article is not talking about the economy as a whole, or about investing, but about high-frequency trading. And trading is a zero-sum game. Value is not created in the 40-milliseconds between trades, and for everyone who makes money trading, someone else is losing money.
There's a good explanation of this in an article by Henry Blodget.
"Go is too hard. I'm going to write my own language."
The key elements in human thinking are not numbers but labels of fuzzy sets. -- L. Zadeh