That's definitely true, though probably only limited to CS (and maybe the top 5% of people going to investment banking). The vast majority of the most intelligent people at any Canadian university will stay in Canada, and given the current political situation, I think that's probably only going to become more true.
Also, anecdotally, I would wager that schools with significant numbers of Americans (e.g., McGill) probably have more US students staying in Canada than vice versa at this point (with perhaps the exception of CS).
Also, anecdotally, I would wager that schools with significant numbers of Americans (e.g., McGill) probably have more US students staying in Canada than vice versa at this point (with perhaps the exception of CS).