The Chinese Y-20/IL-76 flight certainly got a lot of attention in Southeast Asia. I agree with the view that this was practice for a heavy-lift surge exercise. What else could it be? In the event of a military confrontation, this sort of heavy-lift capability would be highly useful in getting men/equipment down to the various bases China has built on South China Sea atolls.
China's other option for a long-range practice flight over water is to the east, where there are plenty of adversarial countries (Japan, Korea, Taiwan) and nowhere convenient to divert if one of the aircraft had an issue. Should one of the aircraft gone tech, a nice atoll base is within easy reach.