How a 20 year old at check in is supposed to understand the daily evolving rules for the 8 other state/territories and police them is patently ridiculous.
They don't interpret the rules at all, the company mandates all passengers have ID checked prior to boarding, the check in officer just goes through the list of pre-boarding items that have to be checked for. Same time they are asking if you are carrying dangerous goods, physically assessing passengers for dangers, such as being intoxicated, aggressive behavior, etc, etc. The airline includes it in the ticket conditions of carriage. Checking a vaccination stub is a no brain activity, they have it or not, not like trying to identify a passenger that could possibly be sneaking a dangerous good on board, be drug affected or be up to no good. If not at check in, do it at boarding.
BTW, rights do not apply to airline travel in regards to this as airlines don't count as common carriers for the purpose of public transport, so they have the right to refuse travel of anyone.