It's my understanding that the figures in the Anna Aero article relate to past bookings - so 52 passengers each day start their journey at BHX and end it at HKG and the other way around. All with a stop somewhere else as no direct flight is available.
This doesn't include leakage to other airports so it doesn't include people in the catchment area who are traveling between the Midlands and HKG but fly from Manchester or London.
The other thing that is interesting to me is that 48% of the people doing that journey from BHX fly on Emirates, which is nowhere near the best route for that journey.
"From Birmingham, Emirates is the leading carrier for connecting traffic, with it flying 48% of indirect passengers between the UK’s second city and Hong Kong"
So if CX started a daily service, for simplicity and they charged the same as the existing players they would probably pick up 50 pax straight away from other airlines + some new travelers who we only going because the route existed.
On top of that they would pick up passengers going to other airports, some of those would be their own pax but they could also be flying with BA or Virgin or in the future HK Airlines.
Plus CX is feeding its own hub so they would also take some passengers, not included in the 52PDEW going to other destinations, like those flying on the kangaroo route with EK or to other destinations in SE Asia and China.