SJ Lepak, Dr. David C. Fowler was one of my professors in the English Literature department at the University of Washington. He has done a number of scholarly works, especially on older English literature, including both "The Venerable Bede" and "The Popular Ballad." Not one of the "biggies" like Sharp or Child, perhaps, but a recognized authority in the field.
And no, I don't think he was reading too much into it. If one looks up the Language of Flowers, the rose generally means true love, with various colors or configurations of roses (e.g., pink rose bud meaning young love, red rose meaning true love, etc.), "fine tuning" what kind of love is being talked about.
The briar, with its thorny or prickly stem, generally doesn't make it in most books of "language of the flowers," not being a flower as such, but in some of the older ones, it signifies rejection, or at best, ambiguous love or conditional love.
So that interpretation makes very good sense in the context of Barbara Allen.