I’m Canadian and disappointed at how ineffective we are at building successful companies.
NB some actual Canadians in this thread have voiced this possibility.
The idea is precisely that not having SV types around _improves_ the country, i.e. makes it closer to the preferences of Canadians.
And yes, having a foreign tech worker doing 9-to-5 in a large legacy company for thoroughly average salaries is very different from having a SV-style startup culture. There is very little process in Canada to make life difficult for the former style of company, and plenty of process to make operations difficult for the latter.
If not having SV folk improves Canada for Canadians, and hqving SV folks improves America for Americans, then this is just mutually beneficial trade. Efforts to try and stop brain drain still makes sense: it's even better if you can convince the citizens you trained to engage in the economic activity you actually want instead of economic activity that you find undesirable, but if you're unable to convince most of them, letting them go is still better than having them stay and engage in their undesirable behavior anyway.
Compare: if a large minority of Icelanders wanted to work for the Baby (which Iceland doesn't have), theb stopping the brain drain (convincing them to work in the Merchant Fleet) is the best outcome, but funneling them out (training them in merchant navigation and watching them join the Danish Navy) would still be preferable to them engaging in their desired behavior anyway (form their own pirate gang preying on the very Merchant Fleet you're trying to advantage).
Immigrants coming into countries start companies at a disproportionate rate compared to natives.
Other than unquantifiable statements about what "Canadians want" everything you mentioned so far to justify this idea of "canada doesnt care if tech graduates leave" is falsifiable by data.