CX has a two faced policy; they tell you the flight will close 40 minutes before departure and the boarding gates will close 20 minutes before the scheduled departure time. They then keep people waiting on the plane for late arriving passengers as in your case. Before (Oneworld) they would just offload the bags and late comers would be late stayers. Since Oneworld flighst are frequently held to await joining passengers from other flights to the annoyance and inconvenience of people who have been there at the gate on time.
It seems therefore that bums on seats means more to CX than its own stated policy. Selective hypocrisy.