I don't know a ton about a modern airliner but my guess would be that the door between the server compartment and the cargo bay can't simply be opened from the cargo bay, as it would be way too easy for anyone to enter the cockpit.
Luggage is rolled in with containers and then locked in place (to prevent them rolling around), that's what the rails at the bottom are there for. You would not be able to get out again.
At that point, if they can get in there (or anywhere near the door), they may as well leave a bomb or equivalent explosive/incendiary device (a big enough lithium battery should do the trick). Actually accessing the equipment room isn't really necessary.