Fair point on citing Wikipedia.
Here is is a document from US CBP:
https://www.cbp.gov/travel/us-citize...vel-initiative
This document confirms that NEXUS cards are acceptable proof of nationality for Canadians and Americans for land and sea borders, but inconveniently omits the whole category of Canadians travelling by air (although does confirm for Americans travelling by air that just a NEXUS card is OK). I found myself in an uncomfortable position earlier this year of having to travel to the US last-minute whilst my Canadian passport was being renewed, so I looked up the US Federal Register documents relevant to the WHTI and they confirm explicitly that Canadians only need a NEXUS card to travel to the US by air. Armed with this, I went ahead with my US trip and had no problems, either with WestJet or the US authorities. The Global Entry machine at YYZ accepted my NEXUS card (as did the US agent after the machine), and WestJet accepted the NEXUS card on its own.
I’ve done the research. You only need a NEXUS card as a Canadian travelling to the US, and it is all documented under the WHTI. You may encounter ignorant agents, but the law is on your side on this.