Edit: Also, the population of Canada has grown significantly (see: Century Initiative)
As a percentage of total population, net migration is the same today as it was in 2011, and lower than it was 1991-2011.
Canada was dealing with a brain drain well before TFWPs and LMIAs were a thing - all the way back to Thomas Edison's family.
The actual number is 20% of all Canadian immigrants leave Canada within 25 years of landing [0], and this rate is 2x higher amongst Canadian immigrants with a doctorate and is elevated amongst Canadian immigrants classified as "business and finance managers, ICT professionals, engineers, and architecture managers" [1].
And speaking anecdotally amongst the Asian American/Canadian community - most who I've known who landed in Canada as skilled immigrants left Canada for the US on a TN visa once they naturalized (even with Trump in office). In a number of cases, they chose Canada simply because they couldn't make it to America on their first try.
[0] - https://www.thestar.com/news/canada/one-in-five-immigrants-a...
[1] - https://forcitizenship.ca/article/highly-skilled-immigrants-...