There could be thousands of reasons for employers to prefer hiring local than overseas graduates. It all depends on what type of business the employers in engaged in and has no relationship as the graduates capability. If I am the employer who is doing a lot of China trades, it will be stupid for me to hire graduates from Canada or US who got no chance to speak Chinese in the past 4 years. But I will definitely prefer someone who has China knowledges, will you consider graduates from Sheunghai (overseas)?
HK graduates usually have some local knowledge, will ask for less in terms of salary, and is well engaged in the local environment. They may be good in Chinese but not so good in English. They are relatively more tends to obey orders.
Overseas graduates (USA and Canada or England) usually have very much western cultures influenced and may have a higher expectation in terms of salary, will have better spoken English language skills but may not be good in Chinese. But they will be more self motivated and mostly they are self starters and very independent in performing research type assignments. But there is no garantee that they will 100% following orders because they were trained to think outside the box.
In conclusion, graduates from HK or overseas both have their own characters and the examples I quoted are quite general. There are always exceptions. I think a smart employer should evaluate what he needs before pre-judge and should not make such a bias assumption when comes to hiring.
Old saying: There is always good student in not so famous schools and bad boys from the Ivy Lead. If you run into a boss with such a narrow mind -- go somewhere else and find a better boss. He is not the one that you want to/should learn from.