Wow, they really didn't do any research on this.
Amazon's first Toronto office opened almost 8 years ago. I moved to the office 7 years ago, when it was 25 people Growth has been constant or accelerating since then. We now have over 1000 full time employees- mostly devs, but also some AWS and Ads sales folks now too. Every department seems to have at least a couple teams here. There's two buildings, both running out of space.
And it's not like it's all "couldn't get a US visa, send them to Canada" either. Most of those folks, after a year with Amazon, could probably qualify for an L-1 and transfer to Seattle- but they don't. And a lot of the people being hired are local talent.
Like most of my coworkers, I could definitely get an increase in total comp if I moved to a US office. My wife and I don't want to live in America though- the culture isn't one we like, the politics are terrifying, the gun obsession and violence... Toronto's not perfect, but it's where we want to stay.