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. 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.