What I was meaning is that demo's do not catch a lot of hacks..
A lot of the anti cheat isn't so much only to catch cheaters as taking away an easy route.
Screenshot grabbing can be evaded and is commonly done so in cheat systems, however this requires cheat makers to actively do so. But most importantly it can stop people from tweaking their graphicscard or ingame settings to for instance not see foliage or smoke or perhaps see shadows true terrain.
The same with demorecording if a cheater is smart he can play in such a way that it doesn't show well on the demos. But without demos he could use an aimbot and continuously lock onto targets. Beside demos are a very strong tool at times that if you think someone cheated that after watching the demo you see that such wasn't the case. Demorecordings are pretty handy to check if someone is using a certain bug or exploit as well.
A lot of anti cheat measures aren't so much about catching cheaters only, but simply denying some of the easy cheating paths. And allowing actual people to look in to a case, as said before a ladder or league will fall if most teams start to distrust each other. And for that its important beside automatic detection systems, that humans can pass a judgement.