It built most of its decision model by playing itself over "trillions of hands", according to Prof. Sandholm. You could almost think of the nightly reviews as a similar process to a human sleeping: we benefit from memory consolidation, pruning, time to reflect, etc. I don't see giving the computer 12 hours a night to do something close to reflection and self-analysis as "cheating," or even very different from what its opponents did over Mexican food each night.

The four humans were even "colluding" too.