I sure don't remember that explanation from any of the series books, but I'll take your word from it it was somewhere. It still makes no sense to me. You can move gold, copper, bronze, uranium, titanium, plutonium, cobalt, and lead from one "world" to another in metal form, but not iron? It makes no scientific sense.
Even the "body iron" doesn't pass but metallic iron does pass argument makes no sense from a scientific point of view. Assuming that "moving" from one world to another is occurring at the atomic level, why would another "world" care whether a particular iron atom is bound up in an anvil or in the blood?