How can players in CS:GO use movement hacks although the movement is server authoritative?

Hello, I have been learning about Server Authoritative Movement, Client Side Prediction and Server Reconciliation for the past few days because I need to implement it in my own games and I have a question that is really bugging me. Technically, movement hacks should be impossible if the players inputs (WASD, jump etc.) are sent and then processed by the server with the authoritative movement code running on it. I have heard that Valve uses that system for CS:GO. However, I have still heard about people using fly hacks / speed hacks in the game. If you have any information on how this is possible or if it's just a hoax, that would be helpful! I'm not using that information for my own cheat clients though, I just need to find a way to block it. I hope this is the right way to ask this question.