It is because of this elevated status of Moshe, that not believing in Moshe as the adon hanviim (and Toras Moshe) is tantamount to not believing in Hashem. The Gemara’s statement of Moshe mipi atzmo means that Moshe relayed them to Bnei Yisrael in his role of teacher and mashbia.
The parties to the Brachos U’klalos in Parshas Bechukosai were Hashem and Bnei Yisrael, and that covenant embraced what was to be a more extensive Torah Sh’bichtav, referred to as mipi HaGevurah. The Brachos and Klalos in Parshas Ki Tavo is a covenant forged between Hashem, Moshe and Bnei Yisrael that embraced the Torah Sh’bichtav as well as Toras Moshe, Torah Sh’beal Peh, referred to as mipi Atzmo.