I spent 9 years there, left a decade ago. Turnover is high, but this is a gross exaggeration. It is stressful, and not everyone can hack it. I was there during and after the '05 layoff when roughly 2/3s of IT was cut. That's still the worst I've heard of from what I hear from friends that are still there.
Ken G definitely does the "Good to Great" getting the right people on the bus thing, which typically means the bottom 5-10% are cut, but even that was slowing before I left.
The turnover for engineers tends to be lower than for traders at citadel and at a lot of quant firms.
Additionally, the turnover for citadel is not evenly distributed across teams. Certain teams and orgs have a lot more turnover than others. Some teams are made up of people with <=2 YOE at company while others are made up of people with >=15 YOE at company.
Ken G definitely does the "Good to Great" getting the right people on the bus thing, which typically means the bottom 5-10% are cut, but even that was slowing before I left.