I think their assertion is that the release of AlphaGo has actually made human Go players worse at the game, contrasted with chess where most agree that the introduction of Superhuman chess engines has elevated the (human) state of play.
But I don't think there is actually much evidence for that. I'm sure the introduction of AlphaGo did take the wind out of some players sails, who thought of themselves as superior to our best computers, but for everyone else it seems to have elevated the overall level of play just the same as the chess engines have done.