That makes sense. The particular trip where I’m seeing this message is one-way US->YTZ.

From their site:
Canadian and US citizens travelling between US and Canada must still carry a valid passport even when travelling with a Nexus card, except when traveling from Canadian airports with CBP preclearance. For Porter passengers, this applies to those departing Canada to the US from Toronto-Pearson and Ottawa International Airport.
https://www.flyporter.com/en-us/trav...ocuments/nexus

It’s interesting that (unlike the US carriers which seem to be completely unaware), they do address the exception but only provide it in the ex-Canada (with preclearance) direction. From refreshing my memory of the entire thread, it looks like they used to insist on passports for everyone, so interesting that they acceded to only the southbound half of the exception. (Maybe related to new-ish addition of YYZ and YOW transborder service? For most of PD’s existence, anyone leaving Canada with them was doing so out of YTZ without pre-clearance.)

This list:
https://www.cbsa-asfc.gc.ca/do-rb/se...erien-eng.html
would seem to suggest no Nexus kiosk / gate arriving YTZ. However it seems from the discussion at length in this thread we’ve concluded that northbound air entry using Nexus card is theoretically fine everywhere per the regs, not just at specified airports with kiosks. (I thought I remembered using an old-style iris scan Nexus kiosk when last arriving there long ago, but I use YYZ much more often, so that distant memory might be totally wrong.) In any event, at an explicitly participating airport like YYZ or YOW, it seems like CBSA is fine with entry into Canada on Nexus card alone, and it’s just Porter’s policy that is not.

(Edit: removed a comment that didn’t make sense because I was confused. The current GE machines use biometrics rather than scanning any type of passport or card, so the fact that in the past GE kiosks at preclearance accepted NEXUS cards is moot now.)