Well I can assure you that, as a former employee of the Canada Border Services Agency, not only am I correct but the link you provided only serves to confirm what I claimed. Therefore I can only assume that you either did not read the information in the link provided or you are unable to comprehend it.

I claimed that a passport is not a legal requirement for U.S. citizens to enter Canada. The requirement is proof of citizenship which your passport may serve as if you choose that option, but it is not required as i stated.

"If you are a U.S. citizen, ensure you carry proof of citizenship such as a passport, birth certificate, a certificate of citizenship or naturalization, or a Certificate of Indian Status along with photo identification"

This means that you may approach a land border, present some other proof of citizenship, and then gain entry if found eligible. Not only that but you don't even have to present your passport when entering via air port as you are entitled to use any of the other options. You are well within your rights to present another form of citizenship proof at an air port despite having a passport in your pocket, though it may be potentially peculiar or suspicious to do so and it is really the best option in that context. Passports are simply the most common form of ID used at air ports by U.S. citizens due to already having it as required by most airlines. This is mentioned here:

"Whether you’re entering by air, land or water, we recommend you carry a valid passport for all travel abroad, including visits to Canada from the U.S. A passport may be required by your airline or alternative transportation authority, as it is the only universally-accepted identification document."

Additionally, U.S. NEXUS travellers seldom have to use their passport and they usually enter the port via NEXUS machines with their cards rather than speaking to an officer. However, there were many instances when I worked at an air port where the machines were not operating properly or otherwise and NEXUS travellers were admitted by myself or officers using only their NEXUS card as proof of citizenship, as seen here:

"Citizens of the U.S. who are members of the NEXUS program may present their membership card to the CBSA as proof of identification and as a document that denotes citizenship, when arriving by air, land, or marine modes."

So, please, if you still believe that I am incorrect and that having been employed by said agency does not mean I am likely knowledgeable in this area then I urge you to provide evidence that actually proves me wrong instead of evidence that supports my claim.