Iḥtibāʾ describes a seated posture in which the knees and back are drawn into a bound or gathered arrangement with a cloth or comparable support. It is a configuration of several elements, not merely the act of sitting.
The expression summons this whole body shape and transfers it to the rein and saddle. Its force depends on seeing how the lines and positions correspond across the two scenes.
It would therefore be incorrect to say that the horse is compared to “sitting down.” What is made vivid is the arrangement of the tack, not the horse itself adopting a seated posture.