Strictly speaking that's insufficient as crossing from the US into Canada by land doesn't invalidate your I-94. They do need to join it against a bunch of other data -- which they do have.
Exactly. They have the data, but they’d need to link it to a few other sources for it to be complete.
The US, unlike Europe (at least in my experience) doesn’t check passports on the way out. They rely on flight manifests or information from Canadian authorities on who has exited.
