John Gill Commentary Deuteronomy 29:16

"(for ye know how we dwelt in the land of Egypt, and how we came through the midst of the nations through which ye passed;" — Deuteronomy 29:16 (ASV)

For you know how we have dwelt in the land of Egypt
How long they and their fathers had dwelt there, the number of years they had been in the land, as the Targum of Jonathan, which was upwards of two hundred years.

Being a country the inhabitants of which were much given to idolatry, they had seen many of their idols, and much of their idolatrous worship; and their hearts had been apt to be ensnared by it, and the minds of some tinctured with it, and the remembrance of it might make ill impressions on them.

This covenant was made to remove or prevent this.

and how we came through the nations which you passed by ;
as the Edomites, Ammonites, Moabites, and Midianites, as Aben Ezra observes, through whose borders they came, as they passed by their countries in their journeys in the wilderness.