«وَإِذَا خَلَوْا إِلَى شَيَاطِينِهِمْ قَالُوا إِنَّا مَعَكُمْ إِنَّمَا نَحْنُ مُسْتَهْزِئُونَ اللَّهُ يَسْتَهْزِئُ بِهِمْ»
When they are alone with their associates, they say they stand with them and are only mocking; God answers their mockery.
The earlier words report what the hypocrites say. «اللَّهُ يَسْتَهْزِئُ بِهِمْ» introduces an independent divine assertion. Incorrectly binding it into the syntactic scope of their reported speech risks confusing who is responsible for the assertion.
Thus faṣl can protect the attribution and independent status of a new statement. The same subject matter need not mean a single coordinate structure.