Thought YUL-LAX was A319 for this specific reason? Long way for the A320, factor in stronger winter headwinds and this won't be such a rare occurance. See JetBlue JFK-LGB for more examples.

Assuming AC will be using 319s for winter season on route.

Think it's a fair question from the OP though. AC knew before departure fuel diversion was very likely. How would the board feel if AC could have removed the chances of diversion by re-routing some cargo (not luggage) via YYZ and decided the cargo was more important than inconvenience of pax? Not saying that happened here, but . . .