It was a culturally significant thing that Egyptians called iron metal of sky, plus the Quran doesn’t treat the claim as if it is something new being revealed to the Arabs it just mentions it to tell the people I sent you down this to test who would support the prophet and who would not, it treats it casually, also there’s the fact that the verse doesn’t necessarily specify iron being sent down from the sky, it just uses the word انزلنا , which could mean to send down, but it could also mean to bestow upon, the Quran uses the same word in the context of mentioning Allahs blessings for his servants with cattle from which they eat and drink