"That is from the news of the unseen which We reveal to you, and you were not with them"—meaning, in their presence—"when they cast their pens," that is: when they cast lots with them.
﴿ذَلِكَ مِنْ أَنْبَاءِ الْغَيْبِ نُوحِيهِ إِلَيْك وَمَا كنت لديهم﴾ ﴿عِنْدهم﴾ (إِذْ يلقون أقلامهم﴾ أَي: يستهمون بهَا.