It's tempting to assume the AI will just keep getting better and better, but that's not guaranteed, and I was happy to see that the Deepmind folks in the video clearly acknowledged this. In the game that MaNa won, it's possible that he did so by finding a strategy the AI agent had never encountered before, causing it to respond with nonsense (e.g. not building a Phoenix and pulling its entire stalker army back to deal with warp prism harrassment). In a game with a strategy space as large as SC2, it's possible that an AI will never be able to saturate the space of viable strategies, and it will always be possible to find edge cases that the AI has no idea how to handle.