Picking out the statistical outliers are not that hard, but will this not have diminishing returns? As soon as the cheaters learns that being too obvious gets you banned they'll change up how they play. Eventually there wont be much difference between the really good players and cheaters, is some false positives okay here?

Many cheaters were already trying to not be obvious, most I've encountered playing various fps games are not the typical spinbot in csgo. Instead they might play with only wallhack, aimtrigger, or even no hack, and only turn on the big hacks halfway through a game if they're not winning or think someone on the other team is hacking as well. In some games they use bots to dunk their stats when not playing.

AI detection is also coming to videogames with anybrain.gg, but seems like these can be countered with AI enhanced cheats no?

As an experienced player with an anti cheat/cheating/security interest it doesn't seem like statistics is the silver bullet you claim it to be, at least as your only detection/protection. It combined with normal protection/detection methods is likely what Riot is doing.