Protests against the government are actually quite common in mainland China these days, however these rarely attract international attention because the vast majority dissidents are actually committed Marxists who think that the current government sold them out chasing after foreign investment and want a return to the "iron rice bowl" of the past. The number of people advocating for a forceful regime change to an Western style liberal democracy are understandably quite few, not because people are terrified to express that idea, but because they've seen the track record of American interventions abroad and want no part of that. Hong Kong, because of its colonial history, obviously has a different perspective on this than the rest of China, but I would definitely be wary of taking their claims at face value.