It is almost certain that not all of the congregation had been part of Alma’s group. We will see Alma visit the descendants of the people of Limhi who had also been in bondage. Therefore, this is a generic call to remembrance of bondage. It is not that all had relatives who had been in bondage, but rather that, as members of the church, their spiritual father, who had created the concept of the church, had been in bondage.
Why should they remember that they had been in bondage? The intended parallel is between physical and spiritual bondage. While the current congregation might not be in physical bondage, Alma is declaring that they are at least in danger of spiritual bondage. For their fathers in the land of Helam (see Mosiah 23:20), and now for them, the secret to salvation was in a change of heart.
The people of Limhi, more than the people of Alma, were in bondage due to their own sins that had removed them from the protection of the covenant of the land. The return required that they turn from that darkness to light, and when they did, Jehovah delivered them.