Generality does not mean that a term applies to everything. Every general expression has a defined domain. For example, “Muslims” includes people who meet that description; it does not mean every human or creature.
The binding scope of an unrestricted ʿāmm is not abandoned merely because an exception can be imagined. Certainty here concerns the force attributed to that expression within this legal-theoretical approach, not a claim that all word meanings and applications are beyond scholarly disagreement.
Takhsis (specification) requires an admissible proof removing some individuals or a class from the original scope.