TWOV applies?

While I figure out my own entry issue, for my husband and daughter who only hold US citizenship, does the following itin qualifies for TWOV?

Fly from US to HKG Stay 1 night in HK Take ferry to Shenzhen arriving at Shekou port (TWOV clock starts) Stay 5 nights in Shenzhen Take ferry/MTR back to HK and stay 3 nights Fly from HKG to Vietnam wi the Vietnam visa (TWOV clock stops, total of 8 nights between entering China and leaving HK) Stay in Vietnam for 5 nights Fly back to HKG then fly back to US (only 6 hours in airport)

If my math is right, TWOV applies as its total of 8 nights from entering Shekou Port to flying out of HKG (to Vietnam)

Thanks in advance!