the idea of Cellular memory is that everything we have ever physically experienced in life is recorded at the cellular level so if you erase our ind, as Paragangians easily do, you cannot erase these cellular memories, thus, things of real significance which you have experienced you will never forget.
Thus, Harris knows on this level who Jason really is and who Desmond really was even though his memories of those facts have been erased.
In a similar way he knows he has a child with DaraJeen even though He has no knowledge really of the existence of this child.
The way it works in this story, I'm not sure it can be cellular memory as such, so it might be more like spiritual memory---[angels above us are silent notes taking, and all that] the origin of this concept for the story was a steak president explaining to me how the cells of our body record our sins and we cannot be fully made pure until the resurrection.