"Tetrachord" typically refers to a series of four notes separated by a perfect fourth, not simultaneous notes, coming out of the music theory of the ancient Greeks. Although that may not be the intended meaning here.
The quote is not calling a major chords a tetrachord; it's comparing major chords to the set of tetrachords (whatever the quoted person has in mind; the linked paper isn't immediately accessible to me), despite not being a member.
The quote is not calling a major chords a tetrachord; it's comparing major chords to the set of tetrachords (whatever the quoted person has in mind; the linked paper isn't immediately accessible to me), despite not being a member.