parabellum,
I know nothing about this particular flight. However "Actual" time of departure has nothing to do with it. The FDP clock starts at sign on, which is sched departure minus 70 minutes in HKG.
The Scheduled FDP for CX830 is 16:10, and this flight was 2:43 late on blocks in JFK, so the actual FDP would have been 18:53.
Max FDP is 17:15, plus 2 hours of discretion = 19:15. So it looks like this particular flight was within this limit. Pretty long day though...