Alma begins a fairly extensive discourse on the Messiah, obviously prompted by Corianton’s departure from the faith in which he was raised to dally with apostasy.
When Alma teaches Corianton about repentance, he does not need to tell him that the Messiah will come. If Corianton is at all repentant, he understands that he must return to this belief. What Alma needs to do is make sure that Corianton understands the true process of repentance, a process that cannot be understood—indeed, cannot occur—without the Atoning Messiah’s mission.