That is a reference to what has preceded regarding the news of Zakariya, Yahya, Maryam, and 'Isa, peace be upon them. It means that this is among the unseen matters that are known only through revelation. If you say: Why did you deny the witnessing, while its absence is known without doubt?

And you left the denial of hearing the news from its guardians, which is imagined? I say: It was certainly known to them with certain knowledge that he was not among those who could hear or read, and they were denying the revelation. Thus, nothing remained except witnessing, which is extremely improbable and impossible. Therefore, it was denied in a mocking manner towards those who deny the revelation, while they know that there is no hearing or reading. An example of this is (And you were not by the western side), (And you were not by the side of the mountain), (And you were not with them when they all agreed on their matter). Their pens are their lots, which are the arrows they cast into the river to draw lots. It is said that they are the pens with which they used to write the Torah, and they chose them for drawing lots as a blessing when they disputed over it, competing in taking care of it.

If you say: (Which of them will take care of him?) What does it relate to? I say: It relates to an omitted phrase indicated by 'they cast their pens,' as if it were said: They cast them to see which of them would take care of him, or to know, or they say.