Alas, NATO stands for North Atlantic Treaty Organization - the A doesn’t mean American. That’s why Canada and the UK are members but Australia and New Zealand aren’t.
Likewise, EU membership requires being a European country. There is precedent for this, as Morocco was denied when it applied to join for that very reason: https://www.latimes.com/archives/la-xpm-1987-07-21-mn-5339-story.html / https://archive.is/hlOdB
Still, maybe Canada will luck out and that Europe thing will turn out to not actually be a requirement for joining Schengen or to join the EFTA. One can dream.
So I just answered my own question. I was confused by this,
Because I thought the Safe Third Country Agreement allowed them to be sent back with no right to be heard for asylum (unless they stuck in and evaded detection for 14 days).
However, according to https://www.wilsoncenter.org/blog-post/safe-third-country-agreement-expansion-causes-asylum-seekers-explore-new-routes
So I guess to get into Canada they’d leave the US and fly in from another country somehow.