He means: "O Jaz'u." And as the other poet said:

You have lied, by the House of God, you shall not marry her,
O sons of the woman whose horns have turned gray, who ties her camel's udder and milks! (1)

He means: "O sons of the woman whose horns have turned gray." What entangled him in reciting that—by placing a fathah (accusative) on the kāf of "Mālika", according to the meaning I described—was his confusion in directing the orientation of His statement: {You alone we worship and You alone we ask for help}, alongside the genitive case (Māliki) of {Master of the Day of Judgment}. So he thought that the meaning of that would not be valid following the genitive case of {Master of the Day of Judgment}, so he put "Mālika Yawmi 'd-Dīn" in the accusative case so that {You alone we worship} would be addressed to Him. It is as if he intended: "O Master of the Day of Judgment, You alone we worship and You alone we ask for help." Had he known the interpretation of the beginning of the Surah, and that {Praise be to God, Lord of the worlds} is a command from God to His servant to say that—as we mentioned earlier from the report on the authority of Ibn ʿAbbās: that Gabriel said to the Prophet, peace and blessings of God be upon him, from God, May His Remembrance be Exalted: "Say, O Muḥammad: {Praise be to God, Lord of the worlds, the Most Compassionate, the Most Merciful, Master of the Day of Judgment}," and say also, O Muḥammad: "{You alone we worship and You alone we ask for help}" (2)—and had he understood (3) from the Arabs that it is their custom when narrating or commanding the narration of a statement that follows speech to address and then inform about an absent party, and to inform about an absent party and then return to direct address, because of what narration of speech entails of third-person and second-person meanings, such as their saying to a man: "I said to your brother: If you were to stand, I would stand," and "I said to your brother: If he were to stand, I would stand" (4)—the way out from what he found difficult regarding the genitive case of "Māliki Yawmi 'd-Dīn" would have been easy for him.