I used to play competitive dota (the original wc3 map) back in the day and the inhouse leagues were a massive pain in the ass. Same with all the various leagues for counter strike (1.6 at the time). UX was terrible, the moderation sucked and you still had to have anti-cheat as people would cheat even in private spaces. Usually the easiest way to get up the ladder was do something dubious and be good friends with the mods so they would rule in your favor.
People use cheats even at LAN events too. So yeah for competitive multiplayer anti-cheat is pretty much required.