I don't know if the indications for registration and use of the cymbelstern were indicated by Marpurg or the English editor of his pieces.
The piece is in two part: Part 1 is called Pastorella
The second part is the harmonization of the chorale by the composer.. I presume Marpurg added it to the pastorella as a suggestion for one or more verses sung by the congregation..
All in all it is a nice piece and probably, if the indications are by Marpurg, meant as an example for use in services