My reference to Tiananmen wasn’t implying that people are ignorant of it, but rather that it can’t be discussed openly in a public forum.

They can and do. Whole books are written on the subject. But, again, there's a question of perspective. You're not going to get shelf-space at a Chinese book store selling something titled "How the Deng Government Destroyed China: The Tienamen Square Incident and the Death of Chinese Democracy" for the same reason you're not going to get Barnes & Noble to host your copy of "George Washington: America's Hitler" or convince Ron DeSantis to put a copy of Das Kapital in every Florida student library.

Write an analysis of it on Weibo that criticizes the government and see where that gets you

The post will be taken down and your account will likely be suspended. But then you'll get the same treatment on Twitter trying to post the word "cisgender". Does that mean Americans simply don't know about transgenderism? Or does it mean moderators for an arch-conservative are running the largest social media site in your country?

opposing viewpoints aren’t actively suppressed by the government

Tell that to Edward Snowden and Julian Assange. Tell it to Bong Hits for Jesus guy. Tell it to folks trying to distribute voter registration information in Florida, Idaho, Kansas, Missouri, Montana and Tennessee.

Equating the two is off by several orders of magnitude.

One of the problems with discussing any kind of Chinese censorship in the United States is that Americans assume they are uniquely privileged and that Chinese people are exceptionally surveilled and repressed. This is largely because the folks making the assertion are saying things that would get you censored in China, but not in the US. By contrast, there are people in the US who have lost their jobs, their property, and their freedom as a consequence of verbally challenging the US government. They aren't able to voice their grievances because they are censored for saying the things you are not allowed to say. They could say these things in China without consequence and quite a few people in China do espouse these views. But where as boycotting Israel or burning an American flag won't cost you your career in Chinese state government, like it will in Texas or Florida, we don't consider this an essential freedom.

The specific set of taboos are different in our two countries. But political taboos exist in both and trigger reactionary responses in both. This does not preclude having a conversation about certain events. But it does shape how people view these events and who they assume are righteous relative to who they assume are tyrannical.

You can talk about the J6 riots around the US Capitol. But you can't run for office as a Democrat and say "The J6ers were right and justified in their actions, I support them!" because the party will kick you out. You'll be excommunicated from Dem social circles. You won't be able to get contracts from Dem officials in blue states. Anyone offering you a job in these circles will fire you. Dem media won't host you on their shows. Your "social credit score" will be in the trash. Does that mean the Dems have censored you? Or does that mean you've alienated everyone you need to do business with the party?