Why do some grammatical analyses posit an omitted complement of yaʿlam in 100:9?

Supplementary Practice 25 — Comparative Rhetoric of Sūrat az-Zalzalah and Sūrat al-ʿĀdiyāt

• The understood meaning and context justify supplying the unexpressed complement
• Because every imperfect verb must have an omitted object
• Because the oath response is never expressed
• Because buʿthira is synonymous with yaʿlam