Or to hedge against the US immigration policy that could hurt their labor supply in the long term. It was not lost on me that half my engineering grad school classmates wouldn't pass the Bannon/Miller test.

I wouldn't bet the farm on this.

Toronto has a better infrastructure than most US cities on the list, saner immigration policies, wages low enough (even if you compare the total cost of employment) to possibly make up for any tax discount. Canada's image is good among European and East Asian tech workers willing to relocate. And its tech scene, while not huge, is decently sized.