This is not accurate. They are not compliant with the residency obligation, and having a valid PR card does NOT change that.
It is true that those with valid PR cards are much LESS likely to have detailed interviews or get questioned about the residency obligation. But less likely does not mean 'no issues.' The issue remains.
Now, it is true that they WILL be admitted into Canada. No question. But they can be reported under the 44(1) procedure which - make no mistake - starts a process (subject to appeal) that leads to revocation of PR status.
Is it likely? Hard to say. But you wrote 'they won't reject you' and while being reported under 44(1) is not 'rejection', it has potentially extremely significant consequences.
And the 'no-one at the airport would know when you left Canada' is a foolish red herring. CBSA officers have access to records, and they can ... ask questions and inquire further if they so decide. If someone has a passport with an entry stamp to home country from ~five years ago, and no entry stamps to Canada in between, that might be sufficient for them to start to inquire in more detail and check those records. (There are some other factors that might influence whether they inquire further on the travel record, like what passport the individual holds, stamps for other countries, etc.)
They may or may not decide to pursue the residency obligation issue. I don't know how likely. But it's not zero chance.