The person/s in the cockpit are said to possess an override button. The door opens only if the override button is not pushed for a certain time after the code has been entered, whether because no one is capable in cockpit or because they do not mind opening the door.
If the hijackers get the code from the few cabin crew who know it, or if the code is simply easy to get, the person/s in the cockpit would simply keep pushing the override button every time the doorbell rings.
Of course, code or no code but a gang of determined attackers with what tools they have found around the cabin could accomplish something by steadily chipping away the door over the 180+ minute diversion time anyway.
Is there a known case of a plane hijacked because the hijackers knew the door code?
Because there is the known case of a plane lost with all aboard since the cockpit was too hard to enter.