Alma reiterates the most grievous of Corianton’s sins. Alma tells Corianton that he should “lead away the hearts of no more to do wickedly.” Following after harlots was bad enough. The real sin was causing others to do wickedly. Alma commands Corianton to begin the repentance process by returning to those he had wronged and to acknowledge what he had done. Why would Alma give this counsel? Because Alma himself had lived it. Alma had done the very thing Corianton had done in leading people to do wickedly (even though nothing was ever said about Alma and harlots). Alma’s repentance led him to a desire to preach in order to recover those he had led away. The sons of Mosiah were led to teach the Lamanites, perhaps because some of what they taught had been an emulation of Lamanite ideas.
Alma knows that Corianton can be redeemed. Alma was. The sons of Mosiah were. They had similarly led people astray. Alma knows the way back through his own personal, painful experience.