The story is hinky. I'll ask someone that knows something about this subject and these machines the facts. Till that time I'm skeptical about it.
I've been in three for sustained physical injuries. Each time I had to remove everything that was metallic and get into either my underwear or that robe they give.
Same with me except that I could wear a wrist watch and kept the steel bridge in my boca with no problem. Steel objects were in the room and nothing went flying.
I used to build the magnets for these things (Stangenes Industries in Palo Alto, Ca.) so I do know a bit about the power of these magnets.
The story is not about the power of the magnets though, but about whether a shielded MRI machine can pick up an object over 2 pounds in weight that is in a holster and fly it across the room so that it strikes the machine with enough force and holds it in such a way that an otherwise safe gun can discharge.
In this case the grip safety would have to have been depressed to allow the firing pin to move forward and the firing pin would have to moving at enough velocity to set off the primer. The hammer was locked back so it did not strike the firing pin.
It is theoritically possible that it could happen with a powerful enough magnet but in this sit I'm skeptical that all facts are there.