How many of the CS students are non-Canadians who came for their undergrad and now moving to wherever they get the best job, or perhaps even move to California for weather?
Edit: I found the answer in the attached document [1]. About half the class immigrated to Canada. The stats for non-immigrant Canadian might be more interesting.
There are two survey questions concerning non-Canadian born students in the linked 2020 profile PDF:
1) International vs Domestic students (6.2% vs 93.8%)
2) Did you immigrate to Canada? (47.56% yes)
You used the answer for 2).
I think the answer for 1) answers your question better than the answer for 2), which doesn't answer when/why they immigrated to Canada.
Plus you're looking at the profile for the 2020 Software Engineering class. Computer science is a different program in the Math faculty. The two programs may have different international student composition.
Tuition is one of the reasons there are so few international students. International students in UWaterloo SWEng pay almost 4x as much as domestic students (C$61.3K vs C$17.1K) - see https://uwaterloo.ca/future-students/financing/tuition.
International CS students pay C$45.5K vs C$9.3K for domestic CS students.
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.
Edit: I found the answer in the attached document [1]. About half the class immigrated to Canada. The stats for non-immigrant Canadian might be more interesting.
[1] https://uw-se-2020-class-profile.github.io/profile.pdf