Must be a safety requirement of some sort. I know other countries around the world where the ovens will be on for take off. Lucky they know the timing (i.e. they're not anticipating any turbulence where they can't actually get up to start the service) so the food - or worse, bread - gets burnt. Do plane ovens have heat safety stop measures or timers?
These days, IIRC the J cabin on 737s is only worked by the CSM, except for the initial set up where possibly another FA helps out. The other 3 FAs on board work the Y cabin. If someone from QF is reading, they can comment better on this (including how fast the service actually goes).
Let's just dissect that a bit.
HKG-TPE is just over the distance of BNE-SYD (by about 40 miles or so). The block time for HKG-TPE is 1h 50min (20 min greater block time than BNE-SYD, and keeping in mind that taxiing at SYD occupies most of the non-air time). It's operated with a twin-aisle, which can be worked with (at least) one FA at a time. That effectively cuts the "service time" in half, i.e. if you had to serve 42 pax, it'd be nearly the same as serving 21 pax in a single aisle configuration.
I don't know about recent times with QF, but barring freak events of overbearing turbulence or the odd oven not working for whatever reason, I've not seen QF have any trouble feeding a full J cabin of pax on BNE/SYD with ample time to eat, on all of 737, 767 and A330 series services (where in the case of twin-aisles, the CSM has additional FAs to assist in serving the cabin; in fact, the CSM is usually relegated to bread basket and coffee/tea duties). With an actual air time of just over an hour, that's ample time to get everyone served. Often, there's even time to do a pass of the cabin to top up glasses.
So I wouldn't be surprised if CX can feed 42 pax on HKG-TPE on a 777 or 330, because that is not a challenge.