[confusedhnguy]

We do know. The number of mainlanders that know about the protests is probably larger than HK's entire population.

If you can read Chinese, go to weibo or tieba or tianya or hupu, you can see posts about the protests. Surely they may get removed after a while but I can assure you that one doesn't gets arrested just for posting the news and discuss about it a bit. The number of off-line discussions cannot be estimated.

Proof (a screenshot from NGA): https://imgur.com/a/FbofCOK

Please remember that HK is physically connected to mainland China and 83.2 million people crossed the border in 2015[0]. Hell, even in 1984 some people still know the government's dirty secrets, it would be extremely arrogant to assume we know nothing.

[0] https://en.wikipedia.org/wiki/Lo_Wu_Control_Point