That is not really a helpful link. It provides the basic rules, not the workarounds when someone forgets a required document. They aren't going to put out a website that says its ok to travel around on an expired passport!

The fact is that if one is a US Citizen, subject to Customs inspection, one cannot be denied entry into the United States so long as on can prove that one is a US Citizen to the satisfaction of the CBP. An expired passport would tend to do that.

And I doubt an airline will deny borading on this account from Canada when one has already cleared CBP before getting on the plane!