One analysis of ʿalā katifihi sayfun treats the initial prepositional phrase as the fronted predicate and sayfun as the postponed subject: the clause is nominal. Another relates the prepositional phrase to an unexpressed verb or participial predicate, which modifies the precise underlying structure.
This disagreement matters. On the first reading the omission of wāw is evaluated within the rules for a nominal ḥāl. On the second, the syntax is tested through the posited verbal or descriptive relation. Each account must be assessed against what the actual context supports.