I doubt there’s actually a defense system. The only way inside from within the plane is from the cockpit and if there’s an intruder in the cockpit they can already do more damage without needing to go below.
There's a door from the cargo area into the server/electronics area. Not sure if there's a handle on the cargo-side, but the pilot uses the door in the video (opened it from electronics side). It appeared this cargo area is pressurized (door into electronics area didn't appear pressurized, nor did the hatch into the cockpit).