So I just answered my own question. I was confused by this,
If those crossing claim asylum, the RCMP cannot send them back to the United States.
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
Asylum seekers are arriving at airports with tourist visas and petitioning for asylum at immigration offices after their arrival. The number of asylum applications made at airports in Montreal and Toronto have tripled since the beginning of 2023.
Overall, the expansion decreased the flow of asylum seekers coming from the United States directly but has not worked to decrease the flow of asylum seekers into Canada.
So I guess to get into Canada they'd leave the US and fly in from another country somehow.
TIL! Yep, that gives the EU exactly what's needed to suspend them from Schengen.