I thought they meant that no game could go more than 218 moves. I can imagine some upper limit since three-fold repetition ends the game. But it’s a lot higher than 218.

Another relevant rule is a draw after 50 moves without a capture or a pawn move. But yes, the maximum number of moves would be extremely large when both players are trying. Just think of a first 2 moved allowing the king out, an outrageous king march, followed by another pawn move...