Note that GP never stated that. They gave an assessment (correct, in my view) of mainland opinion about the protests, they never said that they subscribed to that opinion.
I've been living abroad in various places for a few years now, and I feel like if someone were to vandalise the flag of my home country in an act of protest I don't think it would trigger much of an emotional response in me at all. No more, probably, than someone vandalising the flag of my sports team (that's to say, not much).
I wonder where this emotional attachment to tribal symbols starts and ends. I haven't done much at all in my life (voted a few times, otherwise just lived within the system) to create my home nation. It's just the part of the world where I happened to be born.
Even if I had fought hard to change my country and later become proud of what it had become, I would question whether someone defacing a symbol of that country would trigger an emotional response.
Am I alone in this? Is this tribalism something which is usually considered innate or learned / forgotten?
Before the 1990s the opinions and actions of Chinese people hardly mattered on the world stage. People living in China had relatively low quality of life, and those living overseas were often neglected or treated with biases. There was a society-wide desire to build a stronger nation.
Many factors contributed to China's subsequent success in this endeavor, but I would say that most Chinese people attribute it to their own hard work. And this is a source of immense national proud. This proud is frequently associated with the Chinese flag and emblem when, for example, Chinese atheletes won medals in international contests. They've become symbols that unite a wide range of Chinese people.
Many Chinese who grew up in this period genuinely felt that they lived in good times, and saw China very favorably. They're also likely in prime age today and form the backbone of "mainstream" society. When the HK protesters operate in a context where almost everything about China as a country is interpreted negatively, their messages immediately lose credibility in the mainland.
This very much depends on a country's history and it is impossible to generalise, although I would suspect that burning an American flag in the US (for example) would not go well with many Americans (and probably same for other countries).
China's current regime is partly a reaction to foreign aggression and humiliation.
An enduring symbol of this aggression and humiliation was Hong Kong itself.
This what makes attacking state symbols (or waving the British/American flags as I have seen some protesters do on TV) in Hong Kong so potent.
I think that for many Chinese seeing people protesting against the extradition bill and for more democracy is one thing (which they may be somewhat sympathetic to, actually) but attacking national integrity is quite another, which is guaranteed to produce general anger.
Either the protestors don't know any history, or have learned such a skewed version of history that they internalize the perspective of the 19th century British victors in an ultimately self-defeating way, or;
They know very well the potency of the symbolism and are deliberately declaring themselves aligned with the Western camp out of political expediency, and elevating the conflict to the level of a battle of spheres of influence.
Neither possibility will be looked upon kindly by mainland Chinese firmly rooted in their own historical and political understanding, because it is against their interest.
However people grew up in China are trained to be punished and even disappeared when they speak or act against the chinese official rhetoric, where as being patriotic in the officially approved ways is rewarding.
This extends to them enjoying watching people get punished and disappeared when they speak or act against the chinese official rhetoric. It's immoral and irrational, but it's also human nature. These brainwashing started even before they have the ability to reason.
The more recent police brutality concerns Hong Kongers because, well, now your home and the subway station you use everyday is gassed. And the government is doing it. It's now beyond just abstract concepts like the rule of law and it's a concern of your personal safety - even if you're not a protestor.
The issue of the Chinese flag / emblem being dirtied / thrown away / etc. during protests may be a concern of many in China... but ignoring the more immediate and practical concerns from everyday HKers is not helpful for anyone. Even from the perspective of the Party, such rhetoric only creates more division and problems down the road.
The PRC leadership is not stupid, either. They see the suspension of the bill as test of whether this whole thing was about the bill or something else. As suspected, it was about something else. Given that, no concession the PRC is willing to make will be enough. This is bigger than HK now and frankly out of HK'ers hands as to how it evolves next.
Suspending the bill with the implication of rushing it through when the protests have ended is not a concession, merely a move achieve PRC goals more quietly. Protesters have recognized and called it out as such. The fact that you don't seem to know that makes your statements suspect.