I think Magneto’s power is to control metal, not just magnetic metal. Magneto is just a misnomer/nickname.

To each their own, but I dislike that kind of DMing in my games. When I DM I try to collaboratively storytell. I make sure that I am aware of what kind of story my players want and make sure my players know what kind of story I want to tell, rather than punish them for stepping outside the bounds of my story.