A - standard metal detectors probably won't work well right at the MRI room door. Some facilities may have a longer hallway for access and putting one there, far from the actual MRI suite, would make a lot of sense (I think I visited one location that had that layout), but not all facilities are laid out in a way that that could work.

B - the nature of how a metal detector works would probably have negative impacts on MRI image quality if it is too close to the imager - even outside the shield room door.

I did a sort of tour of a couple dozen MRI facilities for a couple of years, the stronger ones all have radio-frequency shield rooms complete with metal / gasketed doors that are supposed to be closed during imaging. Actual practice regarding keeping those doors closed was pretty loose in the places / times I was visiting. And, in the article's case it sounds like imaging wasn't in progress so the door was probably standing open...