Because you can't rely totally on comms, so I would guess that they were ordered in to check and confirm the cockpit activity, no avoiding being seen by the passengers I'm afraid.
The Comms were all working. I am sure Cathay (like most airlines) has a simple procedure in place wherby the crew can clandestinely signal to their ops that they are speaking under duress.
It would also have to be a pretty big explosive device on board for it to be a concern to the other aircraft.
But why take the entirely unnecessary risk?
It seems to me that the tactics used by NORAD in this example were not very sophisticated or flexible.