You're moving the goalposts from what the article claims. The opponent's strategy is unknown and unspecified, per the puzzle. The point of the article is that

You're moving the goalposts from what the article claims. The opponent's strategy is unknown and unspecified, per the puzzle. The point of the article is that there is a strategy that never does worse than chance, and may do better, depending on the opponent's choice of numbers. Sure, the opponent can make your advantage arbitrarily small - the article says that directly. But the notion that they are trying to do so is not part of the puzzle, and finding a strategy that prevents them from doing so is not what the author claims to have done.