Welcome to AFF.
If you were booked on a Qantas flight number, then my understanding is that it was wrong for them to decline you access. If you were booked on a China Airlines flight number, then they were correct to decline you access.
In both cases, as a Business Class customer, you can access China Airlines' lounge in Hong Kong instead, which should be any of the 3 Plaza Premium lounges. Admittedly these are generally a bit worse than the Qantas lounge in Hong Kong.
