I dont understand the drama about door security myself.
the cabin crew can punch in an access code to request the door to open.
this triggers a delay timer and a buzzer call to the pilots that the request has been made. at any time during the access delay period the pilot can press a button to deny the request.
obviously the cabin crew will need to talk to the pilots to advise why they want access prior to punching in the code.
no matter what the dialogue between them the pilots are in command of the aircraft and can refuse any or all requests for access to the locked flight deck.
you cannot access the flight deck if the pilots don't want it.
if the pilots decide to turn off the intercom to the cabin they can.
there is no great vulnerability lurking here.