Charles Burney, "A Genral History of Music", 1789, describing the ancient Greek modes:

It is very remarkable that all the ancient modes or keys were minor, which must have given a melancholy cast to their music in general; and however strange this may appear, it is ac certain as any point cncerning ancient music can be, that no provision was made for a major-key in any of the ancient treatises or systems that have come down to us.

That is, Burney had a sense of "minor" that generically included all 15 of the ancient Greek modes. Many writers since have needed a similar concept (and it's completely irrelevant that somebody writing a beginners' book on folk guitar playing may not). Metonymic extension of the word "minor" provides it. Birdseye doesn't have an alternative, and if he did he wouldn't have a prayer of getting the world to accept it.