A confirmatory rhetorical question (الاستفهام التقريري) elicits the addressee's acknowledgment of an established fact. An denunciatory question (الاستفهام الإنكاري) rejects or exposes the impropriety of a claim or conduct. Knowing the answer beforehand is not enough to merge these uses.
«أَلَيْسَ اللَّهُ بِكَافٍ عَبْدَهُ»
Is Allah not sufficient for His servant?
The construction elicits acknowledgment of divine sufficiency. In contrast, “A-ghayra Allāhi tadʿūn?” challenges the calling upon anyone other than God. The essential difference is between obtaining an admission of a truth and rejecting an improper alternative.