The answer is simple: they are Cantonese. They are Cantonese because they are from the Guangdong Province of China. Guangdong is a province of China, and it is where the Cantonese people live.
Now, you may think that you are from Guangzhou, and you may be from Guangdong, but you are not Cantonese.
You are a Guangzhou person.
You are not Cantonese.
You are not from Guangzhou.
Guangzhou is a city in the Guangdong Province.
You are a Guangzhou person because you live in Guangzhou.
You are not Cantonese because you do not live in Guangdong Province.
You are not from Guangzhou because you live in Beijing.
You are a Beijing person.