Mormon tells us that he is ending his discussion of the three Nephites. However, as he ends the discussion he also adds to the discussion.
First he indicates that the three are hidden from the world. They are so well hidden that their names cannot be known. In the context of the whole discussion of naming the church, it is significant that Mormon notes that he cannot write the names of the three. Naming them would give others some power over them, at least to perhaps command their presence.
The following verses play on the hidden/revealed theme. The three Nephites are hidden to the world, but Mormon has seen them. Mormon therefore testifies to their reality, even though they are to be hidden from the world. For both the Gentiles and the Jews equally, these three will continue to be of assistance, and will be “among” those peoples, but will not be recognized.