Burning of books during Persian conquest

Ibn Khaldun, in the chapter “On the Rational Sciences and their Kinds” (al-’ulum al-'aqliyyah wa asnafuha) of his Muqaddimah, says: “At the time of the conquest of Iran many books of that country fell into the hands of the Arabs. Sa'd ibn Abi al-Waqqas wrote to ‘Umar ibn al-Khattab asking his permission to have them translated for Muslims. 'Umar wrote to him in reply that he should cast them into water, “for if what is written in those books is guidance, God has given us a better guide; and if that which is in those books is misleading, God has saved us from their evil.” Accordingly those books were cast into water or fire, and the sciences of the Iranians that were contained in them were destroyed and did not reach us. Why did Caliph Omar burn books in Persia or is it a myth?