زَيْدٌ يَكْتُبُ
Zayd writes.
Zayd bears overt nominative inflection. The predicate, however, is an entire verbal clause, “yaktubu.” Its role as predicate places the whole clause in a nominative syntactic position without an overt nominative ending on the whole clause.
Thus “coordination at the clause’s syntactic position” does not mean that each individual word in the second clause must receive the same case ending. It means that the second clause fills the same structural role as the first.