Having watched the livestream from January[0], the AlphaStar agent was able to create its own individual agents to play against and then learn from in the same way AlphaZero learned from chess & baduk/go historic games. In AlphaStar's case this learning period was described as experiencing the equivalent of hundreds of years of aggregate gameplay.
This allows an agent to adjust to novel strategies not seen before because Starcraft II's number of possible game states exponentially increases as the game progresses; compared with chess and go that have large albeit fixed board states. There's also a competition for individuals who build their own AI engines[1] for Starcraft II much like how Stockfish and LeelaZero compete against one another in their own respective AI chess leagues.
This allows an agent to adjust to novel strategies not seen before because Starcraft II's number of possible game states exponentially increases as the game progresses; compared with chess and go that have large albeit fixed board states. There's also a competition for individuals who build their own AI engines[1] for Starcraft II much like how Stockfish and LeelaZero compete against one another in their own respective AI chess leagues.
[0] https://www.youtube.com/watch?v=cUTMhmVh1qs
[1] http://wiki.sc2ai.net/Main_Page