The point is that the flight was overbooked and that this passenger really did CX a favour by opting for the first class seat.

That's because, under EU passenger rights, CX would have had to pay a hefty amount of DBC (amounting to several hundred US dollars) to any passenger it bumped from that flight.

Also .... shouldn't the crew on the flight ex-CDG have known that F class seat was faulty. I thought that the in-bound crew (ie HKG-CDG) would have filed a report which would have listed any problems with the seating ?