Fun statistical fact: outliers are a sign your sampling methodology is flawed, especially when the outliers are a set of samples and not just a singular data point.

"This jet's speed is an outlier in this set of planes. Outliers mean the methodology must be invalid, so jets can't be faster than planes."

This is nonsense. China and the euros have fundamentally different political systems, there is no reason to suppose they should have similar outcomes. The whole point of the discussion is that China's system is superior, if you say that any data that supports that is an outlier, and therefore must be invalid you're just presupposing your conclusion.

On your other point about the usefulness of this data: while it is true that there can be many different explanations for the observed results, that just means that we need more evidence to show which system is more democratic, not that this evidence is useless. Saying that people's opinion of their own system is irrelevant is extremely chauvinistic. In the case of China, we can see the massive increase in quality of life of it's citizens, as well as a systematic overview of it's political structures like here. I've also heard the book Socialism with Chinese Characteristics: A Guide for Foreigners is good, but I haven't read it yet myself.

Furthermore, your point about manipulation of public opinion goes the other way, too. Where did the idea that China is authoritarian come from? People going to China and studying what life is like there, or media manipulation? Who do you think is more likely to be manipulated like that, the people living there who actually experience the political structures of China, or rando westerners whos only source of information is capitalist media? A simple poll like this is more than enough to debunk the people who think China is authoritarian based on nothing but vibes from capitalist media.

https://news.harvard.edu/gazette/story/2020/07/long-term-survey-reveals-chinese-government-satisfaction/