No one calls it HongKongese b/c it is not that different, you're exaggerating. Not all people in Guangzhou can speak Cantonese and those who can, even if they are native speakers, might not accent-savvy enough to tell an HKer if he/she tries to avoid some unique phases.

I am not sure why you have such assertion, all the people speaking Cantonese dialect whether from China (including Hong Kong and Guangdong Province), or from Malaysia, Singapore or Indonesia can communicate and understand each other. There isn't any significant difference. [1]

If you really want to talk about varieties of Chinese then its quite a complex topic.

"Classifications of Chinese varieties in the late 19th century and early 20th century were based on impressionistic criteria. They often followed river systems, which were historically the main routes of migration and communication in southern China. The first scientific classifications, based primarily on the evolution of Middle Chinese voiced initials, were produced by Wang Li in 1936 and Li Fang-Kuei in 1937, with minor modifications by other linguists since. The conventionally accepted set of seven dialect groups first appeared in the second edition of Yuan Jiahua's dialectology handbook (1961):

1. Mandarin

2. Wu

3. Gan

4. Xiang

5. Min

6. Hakka

7. Yue

The Language Atlas of China (1987) follows a classification of Li Rong, distinguishing three further groups:

8. Jin

9. Huizhou

10. Pinghua

Some varieties remain unclassified, including the Danzhou dialect of northwestern Hainan, Waxiang, spoken in a small strip of land in western Hunan, and Shaozhou Tuhua, spoken in the border regions of Guangdong, Hunan, and Guangxi. "

[1] https://en.wikipedia.org/wiki/Varieties_of_Chinese

HK Cantonese has a regional accent (e.g turning the "n" sound into "l") and its own slang but every language has regional accents.