I've herd MREs called "C Rations" when I was in the USAF. We used to have them during war games. It was used as kind of a generic term, like Q-tips or Kleenex.

That said however I've never seen one with metal in it. Some do have a chemically generated heating pack. I suppose there could be some metal in them, but I don't know. And I think I remember some things wrapped in foil.

This may be one of those things where something happens, and an incorrect reason for it is spread by rumor.