Having just stated that Hashem spoke with Yisrael directly, how can the Torah now say that Moshe was a medium?
Ramban #1: The current Pasuk is referring to the number of times Moshe ascended and descended Har Sinai prior to the Aseres ha'Dibros. 1
Ramban #2 (citing Ibn Ezra): The current Pasuk is referring to after Ma'amad Har Sinai, when Moshe began to act as the intermediary between Hashem and the people. 2
Ramban #3: Whereas the previous Pasuk was referring to the first two of the Aseres ha'Dibros, the current Pasuk is referring to the last eight Pesukim, which Yisrael were unable to understand when they emerged from the Mouth of Hashem. 3
How does the word "Leimor" fit in?
Rashi, Ramban #1 (citing Ibn Ezra #1), Rashbam and Seforno: It refers to Pasuk 4 "Panim be'Fanim Diber Hashem Imachem .... Leimor". 1
Ramban #2 (citing Ibn Ezra #2): It refers to the phrase (in the same Pasuk 2 ) "Lehagid lachem es D'var Hashem ... Leimor".
Targum Yonasan: It follows "ve'Lo Alisem ba'Har" and translates as 'and you did not ascend the mountain when Hashem said "Anochi ... " '.