Great chapter. It's a great show of metagame (if I can call it that). Can't wait for next chapters.
It's an unconventional usage of the word but I would say that you're right, it applies here. A metagame is any game-outside-the-game; the game they're playing is just "mahjong", so the theoretical goal is just to win. However, gaming the rules of the tournament (the overall competition outside of just a game of mahjong) by targeting your biggest competition should qualify as a metagame. Normally the metagame would be a bit broader and would describe a strategy that works against most opponents that you are playing in general (such as when to call or riichi based on the playstyles of modern opponents) but in this case it seems correct to use it to describe a single-tournament strategy.