My prospective supervisors are not emailing back, should I just submit a PhD application anyway?
I know your feeling, I was in your situation once.

Thing with Asians universities (far east and south east)
They will not reply to your email unless they know you, or even if they are corresponding authors and you ask them something, they will never reply. That is my experience and in general, I think you should meet them through conference attendance and research work.
The University of HK is regarded top tier in the world, I don't know much what is your field, but I assume they get tons of emails.
Generally, for China, if you are self-funded student, they will allow you to apply without supervisor consent, but if you need scholarship, Acceptance letter is MUST have, this is same with MC, Taiwan, Japan, Singapour and others