All the rest aside, US PRs and visitors with non-immigrant visa are required to get eTA. It's very odd that stackystack was able to board plane without it, while presenting himself as non-immigrant visitor to airline. So, it shouldn't have worked if airline detected the fact that his eTA was rejected.
See
https://www.canada.ca/en/immigration-refugees-citizenship/services/visit-canada/eta/facts.html and
https://www.cic.gc.ca/english/helpcentre/questions-answers-by-topic.asp?st=16.7
US Citizens (aside from Canadians) are the only ones who are not required to get eTA to fly into Canada and can simply withhold their Canadian PR status from airline to board a plane, and then enter Canada as either Canadian PRs or US Citizens (many non-compliant with RO Canadian PRs did the latter and shared their experiences, including in this forum).
If what stackystack describes is what actually took place, then either the airline is sloppy, or some major changes took place during the pandemic and political decision could have been made not to enforce the rules as they used to be enforced before pandemic and after the final steps of
Orwellian Triple Hermetic border initiative were implemented.