In «رَكِبَ كَتِفَيْ غَيْرِهِ», the analysis first relates riding (rukub) to persistent attachment (luzum) through the common idea of close physical adherence. The meaning is then transferred into the verb «رَكِبَ».
This explains how morphological derivation and rhetorical transfer are connected. Merely naming the form “derivative metaphor” without showing the comparison at the source-meaning level is incomplete.
Key point: a resemblance associated with the underlying meaning is realized in the derived verbal form.