The text contrasts expressions like «لَا رَجُلَ فِي الدَّارِ» and «لَا رِجَالَ فِي الدَّارِ», focusing on how singular and plural forms interact with the denial of a class's members.
لَا رَجُلَ فِي الدَّارِ
There is no man in the house.
The singular form can express a denial of the class's presence: even one man would make the claim false. But that fact does not imply that every singular noun with al- has exhaustive meaning. Negation, definiteness, and universality are distinct concepts whose interaction depends on construction and context.