The passage from the Shepherd of Hermas discussed at the link you gave has absolutely nothing to do with any sort of proxy baptism for the dead in the LDS sense. The "apostles and prophets" referenced are dead
and are preaching to the saints of the Old Testament in some sort of purgatory or "limbus patrum" (Limbo of the Fathers). His point is that, even though those OT saints weren't baptized, they are saved by virtue of the prayers and "preaching" of the NT church, which is.
There isn't the faintest shadow in that passage of living Christians on earth going through a literal proxy baptism on behalf of the dead.