The horse-and-iḥtibāʾ example requires discovery of an unusual likeness between two configurations. In the travel verse, the likeness between swift movement and a torrent is readily grasped, but the poet’s redirection of “flowing” creates the novelty.
A rare simile and rare handling of an ordinary simile must not be treated as one cause. Otherwise, every demanding image will be mislabeled in the same way.
A full account of metaphor asks more than where a word started. It asks what newly structured scene its deployment creates for the hearer.