وَاعْلَمْ: أَنَّ الْمَعْطُوفَ فِي حُكْمِ الْمَعْطُوفِ عَلَيْهِ.
Translation: Know that the coordinated expression takes the grammatical standing of the expression it is coordinated with.
If the first expression is a predicate, the conjunct shares the predicative role. If it is a ḥāl, the conjunct shares the circumstantial role.
This is a claim about syntactic function as well as case matching.