What does the word "that" (dhālika) refer to in this verse?

Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. It means "this" (Qur'an). [Ibn ʿAbbās, Mujāhid, ʿIkrimah, al-Kisāʾī, Abū ʿUbaydah, al-Akhfash]
2. It means "that", with various opinions as to what is referred to: (i) What was previously revealed to him of the Qur'an. (ii) What Allāh promised to reveal to him, referring to the verse, "We will cast upon you a heavy word." (73:5). (iii) What the people of previous scriptures were promised because they were promised a Prophet and a Book.