As far as I have heard he was encouraged by Kennedy (the interviewer) to copyright it. As you say he doesn't even claim to have actually written the words apart from a couple of lines. The "if my true love can not come" bit. Apart from that the verses are just mixed up a bit and the words "all go together" inserted for "Braes of Balquhidder". I remember reading the McPeake website and his younger family make the claim that the words are his and purely biographical which obviously doesn't hold true at all. Re the whole thing yes I'd agree it seems to be a new tune to an existing but adapted lyric.