According to the FlightGlobal article you linked, the L3 door was opened (the emergency door behind the wing on the left side). All 8 doors on the A321ceo are plug type doors that cannot be opened when the cabin is pressurized. (Note that the A321neo has a cabin flex option for different door layouts)
I only have access to an A320 FCOM:
The aircraft has four plug-type doors that open outward and forward. There are two of these on each
side of the fuselage (two forward, two aft).
They can be operated from inside or outside the aircraft. Normal operation is manual, with hydraulic
Each door has features that tailor it to emergency situations:
- An escape slide stowed in a container attached to the inboard lower side of the door
- A damper actuator that limits door travel in normal mode, but in an emergency acts as an actuator
for automatic door opening
- A slide arming lever.
(Airbus A320 FCOM - Doors - Description - Passenger Doors, emphasis mine)
The following YouTube videos show old Airbus training videos for the A321 and essentially say that the Type C emergency doors (L2/3 and R2/3) are identical to the main doors, except for size (only L2 is larger) and emergency escape slides:
The aircraft must therefore have been depressurized at the time of the incident. During descent, the cabin pressure targets destination pressure +0.1 PSI:
During descent, the controller maintains a cabin rate of descent, such that the cabin pressure is
equal to the landing field pressure +0.1 PSI, shortly before landing.
The maximum descent rate is 750 ft/min.
(Airbus A320 FCOM - Air Conditioning and Pressurization - Pressurization - System Operation)
Shortly before landing, the cabin pressure is therefore only slightly higher than the outside. According to the FlightGlobal article, the door was supposedly opened at 800 ft. Together with the pressure from the nitrogen bottle that assists the opening if the door is armed, this should make it possible to open the door.
Note that there are no centrally controlled locks on the doors. The door locking is entirely mechanical and controlled by the door handle. Instead of pressure locks, there is a Cabin Pressure warning light that warns the crew on the ground in case of residual pressure difference:
Red Light: flashes when one or both engines stopped, the slides are disarmed, and the cabin differential
pressure is above 2.5 Hpa.