Although computers beat humans at board games without needing any kind of general intelligence at all, I don't think that invalidates game-playing as a useful domain for AGI research.

The strength of AI in games is, to a significant extent, due to the input of humans in being able to incorporate significant domain knowledge into the relatively simple algorithms that game AIs are built on.

However, it is quite easy to make game AI into a far, far more challenging problem (and, I suspect, a rather more widely applicable one)---consider the design of algorithms for general game playing rather than for any particular game. Basically, think of a game AI that is first given a description of the rules of the game it's about to play, which could be any game, and then must play the game as well as possible.