Because the Godgiven metals are still present in true mythology. I stated that already. Celestial Bronze and other godly metals are used to create weapons imbued with special powers and similar attributes. Those being weapons like the Three Brother's Godly Spears, and etc.

We meshed the two forms of Celestial Bronze, the majority of it's properties from true mythology, but with the Camp Half-Blood aspect of being able to be disguised as well. We have meshed many of the concepts, but we derive most of the knowledge and mechanics of the Greek Myths from True mythology.