Long before the devastating encounter with Theia, a kind of sorting process had taken place inside the early Earth. With the formation of the iron core, some elements such as iron and molybdenum accumulated there; they were afterward largely absent from the rocky mantle.
The iron found in Earth's mantle today can therefore only have arrived after the core was formed, for example on board of Theia.
If all of the iron that we have available to us is only there because of a freak collusion then most other life-friendly planets in the universe would not have iron. This would severely slow their technological development. No iron age, no steam engines. No horse shoes.