The realization that this statement is true constitutes the beginning of philosophy understood as the quest to discover what Being is, and in a political sense it is the starting point of the quest for natural law. The first man to realize this, whoever he was, was truly the first philosopher.
The factuality of this statement is inherent in the plurality of tribes and their different customs, which immediately demotes one's own customs from their privileged place so far as nature is concerned; anyone who is aware of this plurality cannot, in intellectual honesty, continue to believe that the customs of their tribe are laws of nature. Yet the customs of tribes have an important human role to play in binding a people together and ensuring their continuity throughout time: this is why the philosopher, in questioning the wisdom of the tribal elders, has always been seen as a threat. This was the basic reason for the execution of Socrates by the Athenians.
The Socratic method is based on enquiring into subjects by first establishing what the customs of various tribes say about something, and then comparing these customs and questioning their consistency so that something like truth might be approached. This was a sure guard against chauvinism, but also obviously a threat to the particular customs of the tribe which Socrates inhabited.
But what this enquiry into truth presupposes is the existence of customs which might be compared, as they are the starting point for analysis. It in fact thrives on the plurality of customs, something which the homogenizing effect of modernity has denied us in the modern age; the general decline of faith in our unique values may have destroyed our chauvinism, but it has not led us to truth, but rather to an intellectual abyss.