He references a source for the text. He doesn't imply he got the tune from there - the title mentions a tune but as Steve says that doesn't mean it provided one. His wording is deliberately vague about where he did get it.

Chappell's position that Scottish tradition was overrated runs right through PMOT. He was pretty unpleasant about it.

Karen is researching 19th century Scottish music publishing now (that's the main focus of the blog I cited) but her thesis was on Chappell - she probably knows more about him than anyone.   That blog cites comments from him about the Advocates Library in Edinburgh that imply he had to have visited it in person. How much time he spent in Scotland beyond occasional visits I don't know. In any case, Motherwell's book was internationally distributed and so high profile in the field that Chappell must have had a copy.