Your belief that China is an authoritarian oppressive regime seems to be unfalsifiable. If approval ratings are high it's because the people are being silenced and secretly hate their government. If approval is low it's because they hate their government. If the approval ratings are real then they've been brainwashed by nationalism.

Like everything indicates the approval ratings are real, there were studies conducted by Western universities that didn't have stake in the matter and most Chinese citizens had a positive opinion of the PRC. There's no way to fake this kind of stuff as well as you're describing for decades.

There are various reasons for why I can understand Chinese citizens like China. They're entering the golden age of their economy right now and legitimately have competitive quality of life when compared to Americans. Their country is becoming a superpower and that could be a source of national pride for some Chinese citizens. I think those reasons can explain the results without adding in coercion.