The lāzim is the connected sense ultimately targeted, while the malzūm is the sense from which the mental transition begins. In kināyah the conventional expression first presents the initial meaning and then leads to another through a recognized association.
In “ṭawīl al-nijād,” a long sword-strap provides the starting image. It can suggest a tall person's stature within traditional usage. This is a customary rhetorical association, not an infallible scientific rule about every sword-strap.
Removing the first sense entirely would change the character of kināyah, whose rhetorical effect depends on moving from that sense toward its concomitant.