Borram
Regardless of the number of engines, under ICAO procedures an engine fire requires a Mayday call. Once the fire is out you can downgrade it to a Pan. As this crew had to dump fuel once the fire was out to return to Sydney they have probably delayed the downgrade from the Mayday until it was completed. Aviating, Navigating and communicating also probably played a part in the delay in the downgrade of the emergency.
In the end once the fire is out, it really is a non-event, even with the dumping of the fuel.