People play pure Daemons lists, they don't always win tournaments with them.
There's a Most Winning Faction section to this page which gives you an indicator of what's going on.
https://www.belloflostsouls.net/2019/06/40k-top-list-of-the-week-june-5th-strands-of-fate.html
'Chaos' refers to a combination of
CSM, Daemons, Chaos Knights, and maybe R&H. This means most people who place in tournaments are mixing factions.
It doesn't mean Daemons aren't capable of winning, just that more people who place are using soup. There could be an outstanding
TS list, for instance, and it would not appear here unless a lot of other people are winning with
TS.
This list changes from week to week, it's like a barometer and sometimes you see something surprising pop up. Worth watching.
Personally, I think pure Daemons lists are more suited for narrative or friendly games than tournaments. While they can win against any other faction, it's harder to build a TAAC list with just Daemons this edition, the balance just isn't there and you have to commit to a specific style of play that will not be effective against all armies. The tactical flexibility offered by a
CSM allied detachment tempers this a bit, which is why you see so many mixed factions.