they went back to LA via Guangzhou but did they definitely fly into Hong Kong? I would make sense to fly into Guangzhou because they don't have cross a border and they won't have Hong Kong visited again. And also Shenzhen airport is quite big, they have direct flights to Bali meaning they could have just flown straight there.
So I think I can provide some information about the reason why they go to guangzhou
For HK:
They have to cross the boarder and board the MTR and transit in different station(From Lo Wu to Airport).
I think it takes probably two hours.
For Guangzhou:
teams can board a shuttle bus to guangzhou
(Since I always go to Shenzhen and take the bus to rural town of China and visit my grandparents)
Shuttle bus may be one of alternate path to guangzhou (3-4 buses/one day)
(Or teams may get a task in guangzhou)
Moreover,there is also an option or the teams to travel from Shenzhen to guangzhou
Probably take 1.5 hrs on average
I hope my info helps