Actually yes, it's very common in China to have people maybe mention or discuss the June 4th Incident, in China it's simply called {六四事件| Liù Sì Shìjiàn}. Everyone in China has seen this image, yet still more than 95% of Chinese in a Harvard Study were found to feel positively about the Beijing Government.
Additionally there is very little to no internet censorship in Hong Kong whatsoever due to the One China Two Systems policy. Beyond information of the June 4th incident genuinely being accessible in Mainland China, it's in addition "extra accessible" in HK. Many even speak and read English in Hong Kong due to English colonization so they could look at your post if they wanted.
Wow that's crazy! I didn't know they did away with the two systems policy, I must be crazy, because I went there and it was clearly different in numerous ways from the rest of China. Weird!
I also wonder how many people were killed by police during those demonstrations, I know many Americans were killed and gravely injured by their police forces now and during BLM.
Additionally, if HK was such a good democracy beforehand, why did the "pro democracy" camp only have 29 seats out of possible 70 if they represented the majority of people in Hong Kong? Maybe in the US if we had a party that did away with right wingers and made housing more affordable that would be a good thing, it seems to have been working pretty well for HKers since.