Redaction: Surely we do not have the entire exchange between Sherem and Jacob. At this point, Jacob would not have simply asserted that the scriptures testified of Christ, but he would have expounded them. Sherem has a legal background - not necessary law of the land, but legal in the sense of interpretation of scripture. Sherem has been expounding scripture with his own interpretation, and would not have accepted a simple statement from Jacob as contradiction to Sherem's entire body of teachings.
Jacob is writing of this incident in the past tense, and rather than record the whole discourse, he condenses it to the salient arguments. The audience to whom Jacob is writing does not need the further elucidation of the Messianic scriptures, for however much Sherem would have.