In kaʾanna ʿulūba al-nasʿ ... mawāridu, kaʾanna functions like an emphatic particle of comparison: ʿulūba is its accusative subject, and mawāridu its predicate. The two objects are compared for the visual pattern of their marks, not equated literally.
Fī daʾayātihā locates the harness marks, and fī ẓahr qurdud locates the rocky comparison. Syntax thus clarifies the two pictured surfaces: camel back : hard rocky ground.