Does 72 hour free transit require joint flight connection?

Hi,
My sister and her family will come from Germany to China Mar 19th and will leave to Germany Mar 28th. They will come w/a tourist visa (one time entrance).

I’m thinking to go w/them to Hong Kong for 2 days from Mar 25 – 27. Hence the flights coming to mainland and leaving won't be joint flights.

Would they be entitled to a 72 hour free transit?

Thx,

Michael