They did say in tweets below the picture that this occurred at about 3000 feet, which means that the interior pressurization wouldn’t do a whole lot. I’m not entirely sure of the precise amount of force required to open the door at 3000 feet in comparison to FL350 where there’s a significant difference between 35K feet outside and 8K feet inside but I would assume it would take some effort.
Another concern is that that is an overwing exit, and, to the best of my knowledge, those overwing exits shouldn’t be able to open like that. Unlike the 737, where the overwing exits “spring” outwards when they’re opened, the DC-9s, MD-80 series, and the A320 series have overwing exits that cannot open outwards — rather, they are meant to “fall” into the cabin of the aircraft, and then either placed on the seats during an evacuation or turned sideways and jettisoned outside by throwing or sliding.
If the claim is true that the exit just “fell” off, that means that some of the exterior “seal” on the exit must’ve failed, indicating a much more serious problem. And if the passengers inside the aircraft indeed opened the exit themselves during the flight, this means that they must’ve opened it the exit, held onto the exit somehow, and then threw a 50-60 pound (23-27 kilo) object outside the aircraft and clear of the engines at the back (quite a mighty feat), all while in the air.
Image source: http://www.skyart.com/sold-out/md-82-emergency-exit-door-white/
If you look at this image of an MD-82 emergency exit, you can clearly see the metal “seal” I’m talking about. I don’t believe that it’s possible for that to fail unless there’s a serious issue with metal fatigue or something of the like involved.