First of all, welcome back from retirement!!! ha ha
Second, a game can be 20-0... the first half must be played. So if a team scored 50 goals in the first half, that is "still within the mercy rule".
It's only when 40 minutes have been played that the game is called after a 9 goal difference.
(The other rule, is that with a 5 goal difference, there is no stopping of the clock).