Massive respect, MudGuard!
Rückert is one of my all time favorite poets in any language,
and one of my favorite human beings as well.

Yes, and some of Loewe's most entertaining, delightful songs
are settings of Rückert;
I am partial to one which I started a Mudcat Thread about,
some years back, about the minister's two daughters.

Your link goes to the critical edition of Carl Loewe's
compositions for the voice.
Dr. Max Runge, who did the editing, printed pages of commentary and notes that touch on every song in the edition.
I did note Dr. Runge's remarks on the Biblische Volkslegende.
The editor had more to say about the melody (which he recognized from other songs)
than about the text;
about the words, Runge said that whoever wrote the words
thought it best not to reveal himself,
whatever that might mean.

There was no comment that the song lyric might be recognizable
from any earlier source,
except of course for the Scriptures themselves.

So I guess the mystery is one that will not be solved.