Mind you, Gary is talking about RTW fares originating from Australia. Certain RTW fares are specific to ex-Australia only.

However, I spoke to the AA special RTW desk 3 days ago and she said that she has never heard of such a ridiculous thing.

I think what your agent meant is when you're ready to book in 'A' class and that class of service is full, and its a 2 class flight, you will be downgraded to M class without any refund, because you are not allowed to book in F class. You may waitlist, but if A class is full, it's full.