وَإِذَا خَلَوْا إِلَى شَيَاطِينِهِمْ قَالُوا إِنَّا مَعَكُمْ
When they meet their devils privately, they say, “We are with you.”
Idha is a future-oriented temporal expression with conditional force. Khalaw supplies the condition and qalu expresses the resulting action, associated with that particular circumstance.
اللَّهُ يَسْتَهْزِئُ بِهِمْ
Allah requites their mockery.
This divine statement is not confined to the moments when they privately meet their associates. Fasl keeps it from wrongly inheriting the temporal restriction carried by idha khalaw.