That's not one of the bolts that holds the plug in place when it's closed. That's the top of the tubular section that extends out from the hinge, with the green collar around it that should be attached to the plug (and was presumably torn off by the forces presented by the airflow when the plug popped open in flight). The bolt which prevents the plug opening would have gone through a radial hole in that collar and the tubular section of the hinge. The visible nut and washer there, as far as I can tell, is just to retain the plug on the tubular hinge section when it's fully open (otherwise it would just slide off).
There's a bit of a mystery around the "loose bolts" that keep being mentioned. Just being loose would not have allowed this incident to occur, they have to be completely missing/removed to allow the plug to move upwards and disengage the stop fittings/pads. If they are loose, but still in their correct bolt holes and have the cotter pin correctly in place, they should still function adequately to prevent the plug from opening. It's not clear exactly what is being communicated by the mention of loose bolts being found on other MAX 9s. It's probably not desirable to have them too loose, but shouldn't be imminently catastrophic; the plug does not depend on them being tight to keep it in place.
Edit: That's for the 4 bolts involved in opening and closing the plug, and the nut on the end of the tubular hinge extension. Obviously there are other bolts around, possibly related to the plug and frame, which could be critical if insufficiently torqued.