«لَوْ كَانَ فِيهِمَا آلِهَةٌ إِلَّا اللَّهُ لَفَسَدَتَا»
Had there been other gods besides Allah in the heavens and earth, both would have been ruined.
The condition is a plurality of gods and the consequence is corruption of the cosmic order. A supposition judged impossible is presented with its implication as an argument for divine oneness.
The important issue is the precise inferential relation between absence of corruption and falsity of the posited plurality. Ordinary conditional statements do not automatically possess the same theological and argumentative force.