The source treats the validity of an exception (صحة الاستثناء) as evidence of exhaustive inclusion. An exception makes sense when an initial claim covers a class and a later clause excludes some members from that claim.
إِنَّ الْإِنْسَانَ لَفِي خُسْرٍ إِلَّا الَّذِينَ آمَنُوا
Humanity is in loss, except those who believe.
If «الإنسان» referred only to one unspecified individual, excluding a group from the initial extension would not have the same force. Nevertheless, an explicit exception is not necessary in every example of generality: it is a strong clue among other possible indicators.