It is true that COPR is not a travel document. Using a previous visa is just to get onboard the plane, because it still looks valid for airline.
Then when you arrive at any canadian border, not matter if you have PR card or not, even if you have nothing but your passport, you have rights to enter canada.
For your references:
IRPA 27 (1): A permanent resident of Canada has the right to enter and remain in Canada, subject to the provisions of this Act.
IRPA 19 (2): An officer shall allow a permanent resident to enter Canada if satisfied following an examination on their entry that they have that status.
@Oldmacwright It is not 100% moral to use this method, but it still remains practical for an inland PA with previous visa who has urgency to travel within 2months of landing for a short term. We all know sometimes, situations just don't work for the PR travel document.