Its fringe stuff, though it does reflect growing chauvinist sentiment in the SAR and it’s a long way from the almost painfully civic minded activism I remember from the eighties and nineties, in the days when activists tended to think that Hong Kong could and would serve either as an example from which the mainland would have to learn or as a base for dissidents to eventually bring change to the Chinese political system, or both. It was also marinated in Chinese patriotism; the idea being not to concede a CPC monopoly on this subject. And anyway, the people concerned were and are Chinese patriots, in the old Republican anti-colonialist tradition. So while there was a lot of discontent about the terms on which Hong Kong went back to China, and specifically about the lack of provision in the Basic Law for meaningful popular democracy. But the idea that in principal Hong Kong was part of China and should return to it was almost universally accepted.
By contrast, sentiments such as the above seem to go hand in hand with the use of colonial era flags on various demonstrations. It’s an attempt to racialize different historical and cultural experiences. They are Chinese, those hordes sweeping across the border for various nefarious purposes. We are…something different and superior. To some extent this is the same foundation as pro-independence movement on Taiwan, though there the reference is to Japanese rather than British colonialism.
It would be wrong to say that Hong Kong chauvinism drives the specific instances of protest you see in Hong Kong. The SAR’s own government is quite capable of doing that. But each instance of protest does tend to spread the message of an irreducible cultural divide. And meanwhile the pro-Beijing DAB have picked up the now less contested Chinese patriotism mantle and used it in their progress to become the SARs most electorally popular party.