In logical implication, one meaning entails another through reasoning not wholly dependent on local custom. In conventional association, a community's familiar patterns and signs connect the meanings. Kināyah can employ either kind of intelligible relation, but the strength and operation of the relation differ by example.
The strap example emphasizes custom, whereas the intensified denial in “laysa ka-mithlihi shayʾ” involves a more subtle form of inference. These should not be treated as equally direct physical observations.
This is why knowledge of historical linguistic custom can matter as much as dictionary meaning when interpreting a classical Arabic allusion.