FYI -- when you provide APIS info with WestJet online and choose NEXUS as your ID, their web site tells you explicitly to put in your NEXUS card number, and NOT your PASSID. It separately asks for your KTN. Seemed to work, as when I got to the Global Entry machine, it knew which flight I was booked on (and I only had carry-on so didn't go to a check-in desk first).

I agree that AC should make this clearer.