Having flown cargo for most of my flying career, most of my flying has been a night. I can attest that late at night controllers a sleepy, nodding off, or new guys being trained. This masking technique is and has been used to sneak around other that friendly airspace. There was not significant weather in the area and it would have been easy to slip up behind the Singapore flight and maintain visual separation. I would bet, that simulator was used to practice the intercept, and masking technique. He was knowegleble of the comings and goings of the usual boring day to day operation of the aviation community. The pilot had this in mind for a long long time
If the pilot followed this plan in this flight traffic corridor, wouldn’t the two planes, one closely following the other, be in the sight line of other air traffic?
Even if the second plane was flying dark, if sunrise over the Bay of Bengal was about 6 am local, wouldn’t the two planes be visible from, say 5:30 am local time onward, by other aircraft flying in that corridor?
What are the chances of not being seen from 5:30 to 8:10 am when the last satellite ping occurred?