It was certainly their arrangement; but where did they get the basis for it from? "Traditional" doesn't tell us anything useful, and is far too often the result either of laziness or a wish to avoid having to pay royalties.

Not that I'm suggesting that either of those possibilities is the case here; I just wish the Tannahills had been a little more frank, so that we might know whether we are dealing with a genuine traditional variant or a modern collation.

People who ask (and answer) questions here often don't realise that there is a significant difference between those things. Although it may not matter to them, it's important that any information we provide here is as accurate as we can make it; and, where there may be doubt about something, that includes saying so.