MacGregor said that he had added material. He didn't write any of it, but introduced it from other versions of the song. If I recall correctly he was not a Gaelic speaker, and therefore will have collated verses from available print translations; there are several (there is not a Gaelic version; there are Gaelic versions).

Most English language texts to be found on the web are Lachlan MacBean's, though few sites have the courtesy to credit him. He died in 1931, so his translation will have been in copyright until very recently. 'Scottish Radiance' (link above) not only fails to credit MacBean; it also includes Barry Taylor's midi arrangement without acknowledgment. I wouldn't trust that site as a reliable source for anything much.