Earlier quoted context omitted.
I believe that mainland people have to apply for a special permit to even cross the border into HK. I would expect it to be even harder to live there as a mainlander. So while they may be posing as HK residents on Twitter, I don't think they are in reality.
How old is your knowledge? China changes fast - my impression was that it was quite straightforward to go to HK. At least for people in SZ...
https://www.scmp.com/news/china/policies-politics/article/21...