I still can't work out all this codeshare stuff, I watched the screen at a departure gate one day as it scrolled through 9 different flight numbers for the same aircraft.
Having SQ on the JKT - SYD route is perfectly reasonable as Garuda offer SIN - SYD via JKT and, until the runway is fixed in WIII, SIN - LHR nonstop. They only realised the B777-300 couldn't take off with the fuel load need to go to Europe with the runway in its current state after they had bought the aircraft.