أَنَّ النَّفْيَ فِي الِاسْتِثْنَاءِ الْمُفَرَّغِ الَّذِي حُذِفَ فِيهِ الْمُسْتَثْنَى مِنْهُ
Negation occurs in a so-called exhaustive exception, where the set from which the exception is made is left unexpressed.
In an exhaustive construction, the expression after illā can function as subject, object, circumstantial qualifier, or adverbial, depending on its governing word. Negation covers an implicit range of possibilities; illā retains one member of that range.
“Mufarragh” does not mean the sentence lacks a complete meaning. Context and syntax make the unexpressed general domain intelligible.