Charlie,
RO’s pesak was only for those Sephardim who don’t say a beracha when it’s a minhag (some say לקרוא את ההלל and לגמור את ההלל on the הלל שלם). RG, on the other hand, considered it a combination of Torah and rabbinic obligation in accordance with the Chatam Sofer (Orach Chaim 208) who writes:
“Commemorating the miracles which saved us from death which occurred on Purim, Chanuka, and the days enumerated in the Megillat Ta’anit is certainly mi-de’oraita… However, the quality and amount of commemoration is mi-derabbanan.”
For a general discussion see Yom Ha-Atzmaut and Yom Yerushalayim by Rav David Brofsky on the website of Yeshivat Har Etzion (from which the above quote was taken).