Perhaps it's luck that the British carved out Hong Kong, because if they didn't, the Cantonese would have probably been assimilated in to Mandarin by now.
Your comment really worries me though, because it seems to me you're conflating the subculture of Guangdong with the culture of Honk Kong, perhaps in an attempt to make a tenuous argument that it is in fact Guangdong, like Taiwan, of which HK is but a tiny part, that desires independence. I think that's dangerously disingenuous to suggest that HK's current political consensus is but a facet of the wider Guangdong region's political sensibilities. I don't see any evidence whatsoever to suggest that what has happened in HK would not have also happened to any other port on the cost of China. HK wants independence because it took on the subculture of Britain not Guangdong.
A sample of some recent articles
"China Is Forcing Its Biggest Cantonese-Speaking Region To Speak Mandarin" https://www.businessinsider.com/china-is-forcing-its-biggest...
"Hong Kong education chief forced to clarify controversial comments about teaching Chinese language in Cantonese" https://amp.scmp.com/news/hong-kong/education/article/216738...
"Guangzhou Television Cantonese controversy" https://en.wikipedia.org/wiki/Guangzhou_Television_Cantonese...
And there's also evidence of the CCP re-writing history https://www.nytimes.com/2017/05/05/opinion/ching-cheong-hong...
You've also ignored the point about how HK did not succumb to the Cultural Revolution. I think that this is an important point on the timeline where the culture of HK diverged from the remaining Guangdong region, which is one of the things that give Hongkongese their unique Chinese identity. For example, Hongkongese retained their religious freedom, as well as freedom of the arts, and language.
So are you indeed saying HK identifies more with the plight of Guangdong than the principles of British democracy? To me it still would seem that Guangdong's resistance to Beijing is fundamentally different from HK's, so that it's merely coincidence that both Guangdong and HK have reason to feel tension with Beijing.
I completely appreciate that HK further differentiated itself from the mainland, in the same way Taiwan did, during the Cultural Revolution. And I see how that must fuel HK's desire for autonomy. But I don't see how that gives any more insight into Beijing's view, which is what I'm trying to bring more awareness to. How do we get beyond the black and white perspective that "China Bad, Honk Kong Good"? Or are you just saying that Beijing is indeed acting completely without reason, as if in a deranged authoritarian vacuum? What doesn't anybody want to understand the nuances of the CCP's motives, no matter how much we might disagree with them?
The language seems like a confirmation bias. If you turned the corporate press standard around, you could also have a headline saying the English deepens their imperial domination of Scotland brainwashing the youth with the Queen's English rather than Gaelic. Also, if what China does makes you uncomfortable, you should be out on the street burning police cars for what the Canadians do in the 'Indian' residency schools.
Linguistically, Southern China has many different groups. In Guangdong alone there are plenty of Hakka and Teochew speakers, both of which are languages quite different to Cantonese. Even within the Cantonese topolect there are a significant number of different groups, for example Taishanese, whose speech is not mutually intelligible with Guangzhou Cantonese. Add Guangxi and Hainan and there are even more different languages and ethnic groups.
I think what the original poster was trying to point out is that due to British rule, Hong Kong did not take part in the advancements (and catastrophes) that happened on the mainland over the past century. That is undeniable fact. The implication was that this was a good thing, but i disagree.
Nowadays in the mainland people who grew up in villages hundreds or thousands of miles apart can communicate using their second language - the lingua franca of Mandarin... meanwhile many Hongkongers are stuck only knowing the language of their local town. Perhaps they preserved their local language better than people in Jiangmen (Taishanese) or Huizhou (Hakka) or Chaozhou and Shantou (Teochew), but a downside is they also can't really talk to people from those cities the way the younger generations of mainlanders can.
You gave an argument that is factually wrong.
Cantonese is not a seperate ethnic group. They are still Han Chinese.
Also, the Cantonese and Mandarin speakers can still understand eachother without study (although their speech may sound funny to eachother).
The language differences are enough to give both groups their own identity, but certainly not a new ethnicity.