The Call Centre agent would not know, and I wouldn’t take what they stated as gospel. The airline wants to ensure you have the proper documentation PRIOR to boarding the plane . That’s why there has been postings here of where the airline has a direct line to the CBSA for them to review documents prior to leaving . Agents would have no clue what the requirements are for common law, nor should they have to make that call . That’s entirely up to the CBSA
If the airline boards someone that should not have been boarded , the airline faces a substantial fine .
When I worked at the airport as a agent , on the rare occurrence, on checking in someone to Australia, I’d get a message to call Australia immigration in Canberra to verify documentation PRIOR to boarding.
Sometimes it would just be to verify something, other times it would be to relay to the passengers that they were being denied boarding /visa cancelled and to go apply in person to the consulate/embassy
Once again , airline agents are NOT immigration agents , and individuals should stop acting like they are during this time . They are risk adverse. If they board passengers who continually get refused entry , and the airline continually gets fined , they will be let go. Seen that as well