If a chinese goes to NY, he has to study in English, he won't have the option to study in chinese. He has to adapt to the place he's going to... Granted, he will have more support than he would have in a HK school. I won't disagree that local schools don't put a lot of effort in integrating outsiders into the local system. It's a fair point and they should do better. HK is not a caring or welcoming society for immigrants. They cater for the rich, another legacy from the colonial overlord.
That is an entirely different point than the quality of the education and the deranged claim that HK students shouldn't have access to international schools to leave space for expats. Virtually all countries provide that choice and HK is no different. The lack of support to foreigners in local schools has absolutely nothing to do with where the elite attend school.