Strength doesn't matter here as much as melting point. When they say tougher than Iron, I imagine they mean it ablated less in the atmosphere. I don't know what elements that might mean. Certainly not platinum group metals. Tungsten possibly. Carbon or diamond. Or alien tech.

Edit: "Based on the speed of the meteor and how much of the object burned upon entry, Loeb determined that it must be made of a material that is tougher than iron."

Yup.