The MC already changed the game's story with this battle, since it's supposedly the event where the hero awakened his powers, in which he altered it by interfering with the battle.
And who knows about the implications with his decision in the future, because he's now literally going into uncharted waters head first, while all of his knowledge on the stories and events could already becomes obsolete, since he changed one of the key events in the game.
Of course, all of these would be useless if the author somehow decides to ignored all the changes, and the story just continues like in the game, which means that everything is already set in stone, regardless what the MC is doing.