Eg. The lack of opposition to the idea that water is wet does not imply that water is in fact dry and everyone is just brain washed
There's no world in which no opposition actually exists when there is a little bit of freedom. You can observe it even in small groups where people can express themselves: you won't have a single convergence of opinions on every topic.
Attack their argument on its own merits.
You cannot have productive discussion with people who have no access to different sources of information. The CCP completely controlling education, media and the Internet makes most people in China in a very bad position to access facts versus pure State propaganda. Add to that a strong social pressure to conform and punishment for divergence and yes, you have mass brainwashing going on.
Now, do you not think that we have any relevant prevailing biases, created by our education, media, history, culture, economic system, and government? [1] Ones that are so deeply ingrained in our society, that we don't even notice them?
If we were to apply your rubric, that would make arguments between people from different cultures completely unproductive - with each person hailing from each culture being able to dismiss the other's out of hand.
The argument you are dismissing as brainwashing seems pretty straightforward to me. It's entirely possible, even reasonable for economic stability to be a goal worth pursuing for government. You may find it anathema, but that viewpoint isn't particularly privileged, in the global space of ideas.
[1] That's a rhetorical question - of course we do. They are incredibly obvious to anyone who's lived in a few different countries.
The difference is the key though. In the West we know that we are brainwashed at some level, because we can observe it happening around us with people who share different viewpoints or cultures.
However, in a country where there's only one discourse that is accepted, you can mistake your own brainwashing for the actual truth because nothing is out there to make you think otherwise.
We are all brainwashed to an extent. Knowing that can at least prepare you to bring a good scrub and soap on your washings.
You have to thank disinformation and mass brainwashing for that. The fact that there is virtually no opposition in China says how far they have gone to control people.