The gemara (Pesachim 6) has a machlokes whether one must start learning Hil Pesach 30 days before the chag or 2 weeks before, from Rosh Chodesh. R’ Shimon ben Gamliel’s proves that only two weeks are required from the fact that Moshe started to teach hilchos Pesach to Bnei Yisrael on Rosh Chodesh Nissan. The Tanna Kamma proves that it must be 30 days because Moshe taught the laws of Pesach Sheni on the first Pesach, 30 days beforehand. Asks the gemara: what does R’ Shimon ben Gamliel, who holds only 2 weeks are needed, do with this proof? And the gemara gives an answer.
The Shem m’Shmuel is medayek that the gemara only asks what the RShb”G does with the Tanna Kamma’s proof. The gemara is not bothered by the question of what the Tanna Kamma does with RShb”G’s proof. Why should that be?
I’m going to put the Shem m’Shmuel’s answer in Brisker terms. There are two dinim in the parshiyos given at Rosh Chodesh: 1) halachos that had to be studies and absorbed in preparation for Pesach; 2) halachos that served as the soul, the life-energy, needed to create "Klal Yisrael.”
There is a parallel between Sefer Braishis, which is the story of physical creation, and Sefer Shmos, which is the story of spiritual creation that centers around the geulah of Am Yisrael. The crowning moment of physical creation is, “Vayipach b’apav nishmas chaim…,” G-d endowing man with a soul. The equivalent moment in Sefer Shmos is G-d giving over the first mitzvos on Rosh Chodesh, as it is Torah and mitzvos which are the soul of Klal Yisrael.
The gemara needed an explanation according to RShb”G for why Moshe needed to teach halachos of Pesach Sheni 30 days beforehand. But no explanation is required for why Moshe taught hilchos Pesach from Rosh Chodesh Nissan. Even were it not for the kiyum mitzvah of learning halachos in preparation for the chag, Rosh Chodesh Nissan is the beginning of the spiritual birth of Klal Yisrael, and that process of creation could only take place through the teaching of Torah.