The article is all about the hominem. He says he left because of how he felt. It's entirely appropriate to ask if there might be other feelings involved.

As another reply points out, he didn't manage that much money. Goldman had clients who managed that much money, and he apparently handled equity derivatives for those clients.

I once worked at a company that worked on an internal web app for one of the largest advertisers in the world, but that doesn't mean I "ran advertising for clients with a multi-billion dollar marketing spend."

To be clear, I can sympathize with someone who used to be in the business of working as an agent for his clients, and who is now in the business of executing the same transactions but taking the opposite side and then hedging his risk. I just don't think moral outrage is the right response to a change in the macro situation. One could argue that while financial markets got more sophisticated, Goldman Sachs coincidentally got evil, but I think it's more likely that markets evolved, Goldman evolved with them, he didn't evolve with Goldman, and he took it personally.