Likkutei Sichos, vol. 19, page 42
“And the man Moshe was extremely humble, more than any person on the face of the earth.” Beha’alos’cha, 12:3
If the prerequisite to learning Torah properly is humility, and Moshe was the most humble of all men, then Moshe Rabbeinu learned the Torah the best.
So, when Moshe Rabbeinu transmitted the Torah to us a second time from himself (“mipi atzmo”), not as a direct transmission from G-d, we know that his understanding and his teaching (by extension) is true to Torah and is G-d’s transmission.