The OBD seem to have learned this song off a Pentangle record; certainly it has never been found in tradition with that tune and refrain, both of which were borrowed (presumably by Pentangle) from another ballad, Riddles Wisely Expounded, around 1970. The refrain has been discussed here a number of times, for example: Chorus to Pentangle's 'The Cruel Sister'
The OBD recording was discussed in this thread: 'twasisters' lay the bent to bonny broom, which also includes a pretty exhaustive list of links to relevant material to do with Child #10 here and elsewhere.
My personal feeling is that OBD just mis-heard the lyric, though I suppose it could have been a deliberate change; at any rate it makes poor sense as compared to the traditional form.