I can't speak for Runescape specifically, but I'm assuming that game had players more spread out in the world, and didn't have everyone standing around in essentially the beginner area where all the free accounts get dumped out like PSO2.
PSO2 is in kind of an unfortunate area as far as "absolute" solutions to the bot problem go, because there's so little differentiating a bot from a real player that's just starting the game and intends to play it through. FF14 solved its problem by blocking most social features for free trials (limiting bots to main cities where most of the population isn't), but it only managed that because at its heart it's a subscription game. I doubt PSO2 wants to lock chat behind premium, when being F2P is its whole model.
They could be quicker about blacklisting specific key phrases, but bots would just find different ways to say the same thing, they could blacklist all the key phrases and then the bots could make symbol art detailing their URLs. Once they enter that territory, there really isn't any way to automatically block them, besides blocking specific reported hashes for symbol arts, but they could easily just have each bot place a random shape in the art's background to randomize the hash to get around that, too.
The only real solution I can see that'd work is having more human employees constantly monitoring the situation, which depending on how many you have, SEGA might not care enough to put that extra expense in.