ETAs are never asked for by an airline. They are automatically linked to your passport electronically only. So an airline upon scanning your passport, will see immediately from the Canadian system if you have a valid and active eTA or not.I had to go back to Europe before receiving my PR card. I read from multiple posts that a PR needs to have his/her PR card or PRTD to be able to board a commercial airline. For this reason I asked someone to send me my PR card by post to Europe. It cost me about 18 CAD. It turned out to be an unnecessary cost.
Today I flew back to Canada, from Amsterdam to Toronto via Keflavik, with WOW Air. For both legs of the flight, I was allowed to board simply with my boarding pass and passport. No one asked for an ETA or a PR card.
When landing as a PR, eTA are not instantly cancelled, this process can often takes weeks or months. So the only reason you were able to travel back to Canada is that you were incredibly lucky they had not gotten around to cancelling your eTA yet. If they had, you would have been denied boarding.