I can see your point but I suspect something was wrong with the seats.
For nearly all routes F is only usually oversold by 1 seat so 2 would be exceptional.
Putting you into CX F will have cost BA so there will be a good reason.
I suspect BA thought that this was better than downgrading you to J.
F often flies full from HKG and the word on the street is that the A380 to HKG is doing very well and is often filling up days in advance so unlikely that there would be spare seats to be honest.....
If you left at 2am from HKG then that's a delayed flight as the last flight with CX departs around 01:25 if I recall to arrive at T3 around 06:30......

FD.