Whilst made up of individuals, a Kahal becomes a new entity which transcends the sum-total of its constituent members. A member of a Kahal transcends their individuality to become a part of something greater than themselves
Moshe gathered all of Bnei Yisroel - כל עדת בנ“י and gave over the words of Hashem concerning the observance of Shabbos and the building of the Mishkan.
There is a dispute between the Rishonim on what the nature of the Mitzvah of the Hakhel gathering is. According to one opinion, it is a Mitzvah for the king himself, that he should gather the Jewish people together.
Moshe had the status of a king and his gathering of the entire Jewish people to hear the word of Hashem, is like the Mitzvah of the king to gather the Jewish people for Hakhel, where they too would hear the word of Hashem.