Qad before a past verb can underline an action's completed occurrence. In a circumstantial clause it is explicitly present in wa-qad balaghaniya al-kibaru. The analysis of other examples, including ḥaṣirat ṣudūruhum, may consider an understood qad to explain how the past clause operates as an attendant state.
This does not mean every past circumstantial construction contains an overt qad. One must distinguish an expressed particle, a proposed ellipsis, and the broader evidence that the clause is circumstantial in the first place.