It’s in the article (and every other article written about this debacle). The plug was of course installed when the plane was delivered, otherwise there would

It’s in the article (and every other article written about this debacle). The plug was of course installed when the plane was delivered, otherwise there would have been a door-sized hole in the side of the plane when it was delivered. The bolts of the plug were probably originally installed, but some rework was required in the area during final assembly, which required the plug to be removed and put back, and during that process the bolts were not reinstalled. It’s (lack of) documentation of that removal-and-reinstallation that are the subject of the article we are discussing here.