If we're thinking of the same study, that was because the US attempting to invade canada would be such a drastically unpopular move that it would kick off a US civil war in response. Also Canada wasn't planning to sit back, but to not engage directly / do a guerilla for the actual invasion while supporting the pro-canada/sanity side with intelligence and materiel.
It was never "watch them destroy themselves", the last damn thing Canada wants is a gutted former superpower with an uncertain political outlook next door.