They're probably saying that a human assisted by a chess program (during training, not during actual game) can on average do better than a human who has practis

They're probably saying that a human assisted by a chess program (during training, not during actual game) can on average do better than a human who has practised chess without any artificial analysis/help. They could also perhaps mean that a chess program that takes human input during processing can do better (on average) than one that doesn't. Essentially they imply human + AI is superior to either of them in isolation.