Technically speaking, they haven’t been able to. There’s really no way of stopping someone using an alternate client if it appears to the server the same way.

The only reason video game cheating is more difficult is because it uses custom protocols and message types, and it needs to be reverse engineered. Usually it’s just easier to reuse the existing game client and patch it to report to the server that everything is normal.

It’s why anticheat runs in the kernel now.