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.