Your assertion was that:

> makes it even harder to believe any positive opinions.

Despite acknowledging that conventionally ~50% of the population are pro government. Ergo statistically there are genuine positive opinions, many in fact. On the mainland or in the west, this is magnified by the PRC population / diaspora scale. So why calibrate your belief meter so unevenly as to reject any positive opinions except dogma / feelings when statistically they are bound to exist in massive numbers.

> The basis is that the authorities can lock you up if you dissent. You have no right to free expression and no chance for a fair trial. You know this and will deny it happens.
...
>It simply does not matter.

None of that matters to genuineness of positive opinions. Self-censorship doesn't translate negative opinions into positive ones, it turns them into silence or contrition. Dissidents who get swept up don't do a 180 and enthusiastically praise CCP, they stay quiet or do boiler apologies and acknowledge being "wrong". In PRC: many people voice their negative opinions in a variety of forums because the chances of being locked up with "picking quarrels and provoking trouble" is stupendously small. Creative critiques that circumvent censorship are everywhere, posts get deleted, particularly troublesome agitators get invited for tea, repeat multiple times before state security commits resources. No one denies self-censorship or persecution happens, it just doesn't happen on a pervasive enough scale to meaningfully collapse public opinion where the default assumption should be positive opinions are not believable. You can argue negative opinions are suppressed, and positive amplified, but that doesn't make positive any less likely to be genuine. Indeed one would expect more genuine opinions by virtue of pervasive propaganda. In the west you have manufactured consent forming genuine anti-China opinions, and Chinese diaspora who self-censor due to stigma and social pressure, but self-censoring of pro-China opinions doesn't make pervasive anti-China opinions less genuine.

>That also happened in Eastern Europe

The comment was addressing the history/state of mainland polling, Eastern Europe during the cold war was absolutely not home to a plethora of western NGOs that operated with relatively loose oversight. PRC was, hence decade+ of western institutions surveying PRC before internal security modernized to the point of having tenable grasp on public opinion. Reason why this was even allowed in the first place is CCP wasn't in position to trust its own data and relied on western data / expertise for development.

>covertly to manipulate

US allocating 300s million to anti-China influence operations doesn't mean we discount genuineness of anti-China opinions formulated by western propaganda. One can suggest brainwashed useful idiots are being misinformed, but doesn't mean they don't genuinely believe the propaganda. This applies to Chinese propaganda as well. Lots of useful idiots genuinely support the PRC narrative, but on balance one can argue the Chinese exposed to both east + west are in a better position to make a more informed decision. PRC diasphora who understands state propaganda is witnessing how manufactured consent can porduce equally brainwashed populous via cover manipulation is something I'd hope someone who escaped east europe can identify.