إِمَّا لِلتَّفَاؤُلِ بِلَفْظِ الْمَاضِي دَلَالَةً عَلَى أَنَّهُ كَأَنَّهُ وَقَعَ
Or as an auspicious past-tense expression, presenting the desired outcome as though already attained.
Tafāʾul is an optimistic mode of expression. A past tense can convey a benevolent wish as if the hoped-for outcome were already achieved; the intent is a prayer, not a historical report.
This form rhetorically brings the desired result closer and allows the speaker to voice confidence or hope. It does not license treating every such phrase as an assertion that the event has actually occurred.