One factor I'd like to bring up here is that it's significantly easier for Canadians to work in the US than it is for people of other countries. Canadians have access to the TN visa, which is effectively automatic, while most other countries need an H-1B, which is lottery-based (last time I was in the lottery it was about a 1/3 chance of getting it).

As a result, I think "non-Canadians who came for their undergrad and now moving to wherever they get the best job" is a smaller set of people than you'd think, simply because they're held back by the (Byzantine) immigration system in the US.

Another related factor is that if you go to an American university for undergraduate CS rather than a Canadian one, you get the opportunity to use OPT (https://en.wikipedia.org/wiki/Optional_Practical_Training), which gives you up to 36 months to roll for an H-1B visa. Add onto that the fact that there are so many good American CS universities to choose compared to Canadian ones, I'd expect most international students to study in the US versus at Waterloo if their goal was to work in California.