Thanks to Daniel, The Philemonasmist
, who comments in an email on Venus, the Bringer of Peace
Unfamiliar with the source cited in the Wikipedia article's footnote, I can't say whether that source is in error or the person quoting from it, but in either case, Holst seems to have relied on Leo's text The Art of Synthesis, not What is a Horoscope?. The former is volume IV in his "serious"/detailed series of works aimed at the practicing or professional astrologer, whereas What is a Horoscope and How is it Cast? was #2 in his series of "Pocket Manuals" meant for a more popular audience. After the preliminary chapters of TAoS, Leo dedicates a chapter to each planet, also giving each such chapter a subtitle in the same fashion as Holst would later do with his compositions -- and Neptune has the same title in both the book and in Holst's suite. (The planetary chapters are: 4. The Sun, Life Giver; 5. The Moon, Mother; 6. Mercury, the Thinker; 7. Venus, the Unifier; 8. Mars, the Energiser; 9. Jupiter, the Uplifter; 10. Saturn, the Subduer; 11. Uranus, the Awakener; 12. Neptune, the Mystic. He also had three other volumes, each based on an astrological lecture he'd presented, with titles Mars: the War Lord, Saturn: the Reaper and Jupiter: the Preserver.) The Gustav Holst site indicates that he had a copy of The Art of Synthesis in his library.
Born William Frederick Allan, by the way, he apparently agreed with your idea that Leo's an appropriate name for an astrologer; it's said he chose it as it was his "sun sign".