This seems to be what is implied in Rashi’s commentary on the verse. It also fits in with the Targum’s subtle change in the text: instead of Moses speaking to (el) the leaders, Targum Onkelos says that he spoke with (im) them.
Voice technology had not been invented, so there was no way in which Moses’ voice could carry loudly and clearly enough for every Israelite to hear his words. The heads of the tribes were therefore the leader’s conduit.
He gathered the tribal leaders together, imparted to them the Divine message, and charged them to pass it on to their own tribes.
The procedure was then repeated within each tribe, so that the heads of the tribes assembled their deputies and told them what they had to convey to the people, and thus there built up a whole chain of command. This seems to fill out what is said in Lev. 17:2 and other passages where Moses is described as speaking to the whole people.
This method was utilised centuries later when it came time for Rabbi Yehudah HaNasi, the redactor of the Mishnah, to convey mishnaic teaching to the people. He charged a corps of specially endowed individuals who had the gift of remembering things verbatim and told them what to remember and pass on to the people.
In this fashion the Mishnah was published – but the “publication” was in oral form and only later was its content recorded in writing.