How does the Quran praise Dhu al-Qarnayn as a righteous servant who believed in God, while Greek historians unanimously agree that he was a pagan?

Dhul-Qarnayn, mentioned in Surat Al-Kahf, was a righteous king who worshipped Allah. There is disagreement as to whether he was a prophet or not. He is not Alexander the Great, and the reported Hadith that he was the one who built Alexandria is not authentic. Dhul-Qarnayn, mentioned in the Quran, lived during the time of Al-Khalil (Abraham), peace be upon him, whereas Alexander the Great lived approximately three hundred years before Christ, peace be upon him. Greek historians did not mention Dhul-Qarnayn, whom the Quran speaks about.