That actually seems like a pretty robust design, and a good way to have a plug in the hole while leaving the fuselage able to have an actual door option. I'm also reading that this door plug design is used on all the 737 NG planes, not just the Max's, which lends it some more reliability, if accurate.
That's contrary to what I've read -- which is not surprising.
737 Max aircraft with larger seating capacity use that opening (and the one on the starboard side) as actual exit doors, not plugs. Some airlines that don't need the exit doors there nonetheless order their planes with the door, but deactivated. Those don't have a regular cabin-size window in the door, they have a smaller, round porthole. I believe the models that have deactivated doors also have "Door Open" sensors for those doors connected to warning lights in the cockpit.
Apparently the models with the plug don't have sensors on the plugs. If this plug had been working its way loose during previous flights, this would explain why the only warning the pilots had was a non=specific loss of pressure warning rather than a specific "Door Open" warning.
