That may be true in some cases, but in many other cases the AI really does cheat
My answer did not imply that the AI doesn't cheat :-/
The interesting questions here involve the perception of fairness and the illusion of competing with a more-or-less equal in single-player games. When people say the AI cheats they mean that it's not bound by the rules applied to the human player, but why should it be? Consider MMORGs -- do mobs cheat, e.g. by using abilities that the player does not have? Do raid bosses cheat by having a gazillion HP, gaining temporary invulnerability, spawning adds, and generally being a nuisance?
In MMORPGS, the game and setting are usually asymmetrical by design - there's no assumption that the human knight should have an equal amount of hit points as the ancient dragon, and it would actually violate the logic of the setting if that were the case.
The games where people do complain about AI cheating tend to put the enemies in a more symmetrical role - e.g. in something like Civilization or Starcraft, the game designers work to actively maintain an illusion that the AI players are basically just like human players and operating under the same rules....
So chess and Go are both games of perfect information. How important is it for the next game that DeepMind is trained on to be a game of perfect information?
How would the AI perform on generalized versions of both chess and Go? What about games like poker and Magic the Gathering?
How realistic do you think it's possible to train DeepMind on games of perfect information (full-map-reveal) against top-ranked players on games like Starcraft, AOE2, Civ, Sins of a Solar Empire, Command and Conquer, and Total War, for example? (in all possible map settings, including ones people don't frequently play at - e.g. start at "high resource" levels). How important is it for the AI to have a diverse set/library of user-created replays to test itself against, for example?
I'm also thinking... Shitty AI has always held back both RTS and TBS games.. Is it possible that we're only a few years away from non-shitty AI in all RTS and TBS games? Or is the AI in many of these games too hard-coded in to actually matter? (e.g. I know some people who develop AI for AOE2, and there are issues with AI behavior in the game being hard-coded in - e.g. villagers deleting the building they're building if you simply attack them).