I think the contrast is between active and passive funds.
If your money is in an active fund, there's a manager exerting his intelligence in trying to make good choices with your money. This effort is beneficial, as it helps the market find the right prices for assets.
A passive fund adds money into the system, but it doesn't add any intelligence - it relies on the intelligence of the current market participants.
As more and more money switches from active to passive, we have more and more money relying on less and less intelligence. This means that the market is becoming less and less efficient, and prices are deviating more and more from where they should be.
Passive investors are essentially leeching returns off the work of the active investors.
This article suggests that the effect will be ultimately catastrophic, where I suspect that it'll just result in money slowly swinging back the other way as active funds take advantage of the situation to start to make more money than before. That's pretty much what the article says he's doing.