The TL;DR is that they win on a rules technicality. (I think by using a ruleset which isn't used by KataGo)
> However, KataGo plays a pass move before it has finished securing its territory, allowing the adversary to pass in turn and end the game. This results in a win for the adversary under the standard Tromp-Taylor ruleset for computer Go, as the adversary gets points for its corner territory (devoid of victim stones) whereas the victim does not receive points for its unsecured territory because of the presence of the adversary’s stones.
KataGo has finished securing its territory under virtually all rulesets but Tromp-Taylor. Tromp-Taylor is a fine ruleset (courtesy of our own https://news.ycombinator.com/user?id=tromp), but not one you'd use to, like, play against someone on the internet, or in real life.
When they induce KataGo to play bad moves, like Lee Sedol did in the only game he won against AlphaGo, they'll have my attention.