henryp; But did they not recognise that it wasn't actually a Scots tune?

Tattie Bogle; Correction 1: many people who attend(ed) Scots Music Group got the dots for Ashokan Farewell from one of their tutors’ books.
Nigel Gatherer sought permission from Jay Ungar to publish the score before printing it in said book. So they will know it is not a Scottish tune.

Nigel Gatherer has compiled a large collection of American Music with seven different sections. But Nigel must have thought it more appropriate
to put Ashokan Farewell in The Scottish Collection in the section Airs & Song Airs;

Ashokan Farewell slow air/lament
        
This tune was composed by Jay Ungar in the style, he has said, of a Scottish lament, and descriptive
of his "sense of loss and longing" at the end of one of the Ashokan Music & Dance Camps.

This set is part of the following collections: Nigel Gatherer's Scottish Collection.