Those with US passports can board a plane with no issue. They just have to show the eCOPR (or whatever else, really) at passport control and present there as a PR (showing passport just to confirm ID).You travel with your US passport but you have to request a travel document to come back. The eCoPR is not a travel document as it is boldly writen on it " Not for travel"
Those PRs without US passports need a PR card or PRTD to board a plane to Canada. If they can arrive by land (eg through or from USA), they can enter by a private car or on foot - the eCOPR (or whatever else) will be sufficient to get admitted as a PR (with their passport as simply supporting ID).