I have been doing some (small scale) data-related business in China, and I am actually surprised Signal has been available there until now.
China tries to keep the Great Firewall strong, i.e. the interconnections between China and the rest of the world are well monitored. Moving data across the firewall is both practically and legally challenging. It is not straightforward even to set up a company web page in China. China seems to aim at Internet independence so that it can live without the western Internet, and moving larger amounts of encrypted data is sometimes slightly challenging.
On the other hand, China does not seem to try to block all connections. Last time I was in Shanghai (just before COVID), I was with a group of executives doing business with Chinese companies and government (sometimes it is hard to tell these two apart). Our lovely official guide told us that we might experience some lags or other challenges trying to use western web services. (Sure, I was kicked out of the hotel WiFi for trying to use a corporate VPN over it. That was a new trick but reliably repeatable.) But then she also said that "these restrictions tend to change, and we have learned to live with them, and there are ways to access everything if needed".
I think this statement reflects the general attitude by the government. The restrictions are there to a) remind you to be careful and b) to keep average Joe/Jill from getting lost in the big bad western world out there. They are not trying to make everything waterproof. So, Signal has been under the radar until its usage has grown too much.
(And, no, communism is not necessarily the key word here. It has a lot more to do with the view of individuals' rights vs. rights of the community. Easter cultures may have a slightly different view on that topic.)
Couple of issues there...
1. As much as the great firewall may be to stop "big bad western" influences, its also China blocking access to any criticism. There's a slew of propaganda China is promoting (cultural, historical, political, etc.) among their people. How would you brainwash people if the internet/app eco system was truly open?
2. Equating and/or justifying Chinese communistic/dictator-like practices to Eastern culture is dangerously misleading, not to say offensive to many democratic countries with varying types of the so called "Eastern culture" - the very countries enjoying democracy. Belief in "rights of the community" does not lead to blocking Facebook, Google, twitter, instagram, a voting system that's practically meaningless, a PM with no term limits, and so on and so forth..
Sure, China can't waterproof everything, but they sure are doing a great job at making it quite inconvenient, which is all they need to influence the minds/thoughts of their people. And it's practically impossible to block everything, China just focuses on the big players first where the most impact can be made (its not like China is being generious in letting some services off the hook). Like you said, Signal happened to grow the point of concern, hence the censor.
