So can/does simply transposing a PD melody line into a different key constitute "arrangement" for copyright purposes?

And, can anybody tell me why the Benedictines' Collegeville Hymnal gives copyright credit for the old Dutch tune In Babilone to the Episcopalians' Hymnal 1982 (© 1985), when all they use is the melody line, and that melody line is identical to the one that has been in the public domain for almost 300 years (first publication 1710)? Hymnal 1982 itself doesn't assert such a claim. It may be that "harm. Charles Winfred Douglas, 1867-1944" (in the Collegeville Hymnal refers to an arrangement that is under copyright, but I don't see why that would affect the PD status of the melody line (in G Major) which is all they print. (And all my copy of Hymnal 1982 shows, either. Maybe the copyright arrangement is in the organists' editions?)

Liland