A PRTD is only required for those who are not visa-exempt. They must show the commercial carrier (plane, train, bus etc.) that they can enter Canada; without the PR card, they must get a PRTD or enter by land in a private vehicle/on foot.
Those who are visa-exempt can travel to Canada on the strength of just their passport as if they were a visitor, at least until the eTA becomes mandatory.
There is no requirement to have a PR card or COPR to enter Canada. I imagine CBSA questioned this person for other reasons, such as the possibility of not meeting the RO.