Shooting people through walls/smoke is a perfectly normal gameplay pattern in tactical shooters so it isn't that simple

If the game is designed so bullets can go through walls then one would expect situations where the player can intuit the location of the enemy.

Imagine a tall wall which blocks all vision of anyone standing behind it. Imagine most of of the wall is knocked down flat but some remains standing just enough to hide 1 standing player. Suppose you see a player walk behind that wall and not come out the other side. It's reasonably to deduce you can shoot that wall to damage that player.

Suppose the surroundings are quiet and you can hear footsteps of enemies you can't see in a building. In knowing the area you can deduce where the enemies are depending on sound of footsteps on glass or on wood. Most games probably have directional sound which helps too.

~That helps a bit, but if you watch the video, you can see that the cheats have become so sophisticated that even a server wouldn't be able to track them. Stuff like offloading display output to a 2nd computer, identifying enemy players and spoofing a mouse's inputs via a microcontroller to move your mouse to the enemy as if it were a "real" player.

Anti-cheat in general is simply unable to monitor systems at that level of physical complexity, server or client.