I used to co-run an online gaming ladder back in the Quake 3 days.

There were aim bots and other client side hacks back then but we requested that folks record and upload demos of themselves for competitive matches. This allowed anyone to replay the game from your exact POV in-game, complete with hearing and seeing exactly what the real player saw at the time.

We all survived back then without kernel level anti-cheat tools with a very high certainty that no one was cheating.

Even if you tried to hide it, it was pretty obvious when someone was cheating. I don't recall a single unsolved case where someone was cheating and got away with it while the community really thought otherwise. This was with over 10,000 registered players and tons of active teams playing every day. No where near the scale of gaming today of course, but it's a big enough sample that the method does work for online competitive play.

Nowadays it would be even easier to detect foul play because with live streaming and human announcers, you're under a lot more analysis by the public in real-time.