لو كان فيهما آلهة إلا الله لفسدتا
If there had been other gods besides Allah in them, both would have been corrupted.
Distinguish the antecedent from the consequent. Plurality of gods is the supposition; corruption is the stated consequence. Reasoning from a conditional requires attention to the claimed connection between the two.
If law is restricted to a past historical time, one might mistakenly treat the argument as merely saying that no past disruption was observed. Here its force concerns the relationship between a hypothetical condition and its result. The logical nature of that relationship still warrants investigation.