Earlier quoted context omitted.
I think you've misunderstood both them and the geography of South China. They're clearly referring to the Cantonese speaking region, which is called Guangdong (which the British never owned), of which HK is a very small part (perhaps 1/30th of the size). It sounds like you think they were just talking about HK or that the British owned GUangdong. And even more confusingly attributing HK's linguistic distinctiveness t…
To add to the confusion, there are plenty of people whose families have lived in Guangdong for generations that are not ethnically Cantonese at all. 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, f…
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.