Why is it unsound to classify “a-tunkiru anna al-waʿda qad taḥaqqaqa?” as invariably rhetorical denial?

Supplementary Practice 8: Rhetorical Analysis of Five Connected Passages

• The interrogative hamzah invariably makes it a rhetorical challenge
• Its pragmatic force depends on the context and reply
• A subsequent reply always makes the question a genuine inquiry
• Qad invariably makes the question confirmatory