We know that's from his letter to the Romans, but Cyril is saying this not with a script in front of him, so he's ad libbing. So it was now part of the communion liturgy, and he's saying that this is meant that you are showing you have complete peace with your brother.
Now, perhaps during the communion service at that point in time, you might walk across where the church, the auditorium of the church, where the service was being held, and if you had not been reconciled with a certain brother, maybe you would be reconciled right at that moment. Forgive one another, give a kiss of peace. Join us in earnestly contending for the faith that was once for all delivered to the saints.