The collision avoidance system only works when the transponder is enabled. So flying in the radar shadow of another aircraft would take some extreme skill. And wouldn't they have to adjust their position according to the position of the several radar antennas? What about potential mobile radar systems, e.g., on military ships? It would have also all depended on perfectly finding SIA68. What if it had a two hour delay? Not unlikely on international flights.
Where would they secretly land the aircraft? It takes a more than 1km straight and wide runway. And it has to be remote enough from any town or military installation. But they would have to bring large amount of supplies like jet fuel. (Which they would secretly get where?) They would have to deal with 240 people on board. Someone who would steal such an aircraft is certainly ruthless but ruthless enough to murder 240 people? They would have to hide the aircraft from satellite surveillance. It would take a ground crew to refuel and maintain the aircraft.
All in all there are so many things which could go wrong. And it's very unlikely that a bunch of amateurs could pull it off.