It's not credible that C-level management were unaware. 5000+ employees doing the same thing, repeatedly, means collusion and coordination up the management hierarchy was taking place. Either internal anti-fraud controls failed, or this activity was actively ignored at management direction. At the very least, the C-level manager at the top of the organization that engaged in this fraud was responsible for a system failure and should be dismissed for cause, without their bonus or severance. Having a lawyer say "Huh, I guess they didn't know" is exactly the cover they need for feigned ignorance.