The state of the game, not the behavior of playing it intentionally. There is a world of difference between the two.

It was able to model the chronological series of game states that it read from an example game. It was able to include the arbitrary "new game state" of a prompt into that model, then extrapolate that "new game state" into "a new series of game states".

All of the logic and intentions involved in playing the example game were saved into that series of game states. By implicitly modeling a correctly played game, you can implicitly generate a valid continuation for any arbitrary game state; at least with a relatively high success rate.