Does this work both ways? Also Hong Kong?

Our friends from the UK are planning to visit us in Shanghai. Can they stay either side of a break to another country, for example:
- Fly UK to Shanghai
- Stay with us for 4 days
- Take a one-week trip to Vietnam
- Stay with us for 4 days
- Return to UK
That's 8 days total in Shanghai but it's in two blocks of 4, each in transit - does it work?

Also - From rules above looks like it works for Hong Kong too?

Many thanks for your guidance