Possibly because the flights are operating as OX, and they have a certified copy of the OX AOC (no details required) on board?
I apologize for my ignorance, however I have to ask how is possible that an AOC is valid in one country and not in another one? Is clear that the EU is not recognizing the AOC issued by the Indonesian CAA, however here we are talking about OX using OG planes and crew. OG has not a valid AOC, therefore HKG should be aware of it.