Why was the word "كل" (every/all) in the verse "تذهل كل مرضعة" (every nursing mother will forget) in Surah Al-Hajj in the nominative case, despite referring to an object?

The reading of the majority [of reciters] is: (تذهلُ كلُّ مرضعة) with a fatḥah on the tāʾ and rafʿ on kull. The "nursing mother" is the one from whom astonishment (ذهول) emanates upon seeing the earthquake or the Hour. It is also said that the meaning is: "Every nursing mother will be astonished away from what she nursed on the Day you see the earthquake of the Hour." Another reading is: (تُذهِلُ كلَّ مرضعة) with a ḍammah on the tāʾ, a kasrah on the hāʾ, and naṣb on kull. Its meaning is: The earthquake or the Hour will astonish every nursing mother.