ثم قدّم أحوال الإسناد على أحوال المسند إليه والمسند مع تأخر النسبة عن الطرفين
The states of predication are treated before those of subject and predicate, although a relation is conceived after its two terms.
In forming an assertion, one can first apprehend the two terms and then their relation. But for instructional organization, isnād is foundational: a term functions as musnad or musnad ilayh through its role in predication.
The order in which something comes into existence or understanding is not necessarily the best order in which to teach it. The discussion begins with the relation because it illuminates the roles of its two sides.