I believe this particular strip is referring to a period of time when a smog in Beijing travelled to Hong Kong causing extra air pollution.
I live in Hong Kong too so I definitely agree that we get some clear days. Here is a picture I took just 5 minutes ago:
It’s still a bit cloudy but you can clearly see that the sky and the sea are blue!
I can assure you, however, that the incident illustrated and Hong Kong’s air pollution issue were not made up. In fact they were covered by several Hong Kong media outlets, and here are a few I found (in Chinese):
Apple Daily: [x][x][x]
Sky Post: [x]
Bastille Post: [x]
Hong Kong Commercial Broadcasting [x] (this one actually shows that backdrop)
I do agree that the Daily Mail article is not the best written one on the subject though. I only sourced it because it’s where the image is from. If you know where I can find a better English article with image, I’d really appreciate it if you can let me know!