Being opensource doesn't imply the game server will allow modified builds to work. Being able to see the code doesn't mean you will find a suitable way to circumvent the anticheat.
That being said, I agree it would be harder to maintain an open source anticheat effectively.
How would hardware attest to the game server that the player cannot see through walls, and that their aim is not nudged (subtly or overtly) in the direction of enemy faces? Keeping in mind that the cheater has full control over the software running on their computer, so they can decide what to send to the server. Also the cheater doesn't need to alter the game itself, they could access the game memory and implement their aimbot by having a clever mouse-driver.
