> true sandboxing if implemented correctly could prevent cheats in the first place

Absolutely not: Sandboxes keep things in, not out, and cheaters are typically the lawful root-administrators of their machine.

So they just run the games sandboxed and launch their cheats in a more-privileged mode. For example, a cheat can read the game-memory and provide a "radar" overlay to help the user locate hidden enemies. More advanced versions can could replace game-code (from the outside) or even tinker with the network-stream.

The game, meanwhile, would have no recourse, since it would (correctly) be unable to see or influence anything outside of its sandbox.