Mormon takes a short break from developing his historical story and inserts the moral of the lesson he is attempting to show in this break between wars. Not only do they have a time of peace, but it is also one of prosperity. This is exactly what the covenant of the land promises, and Mormon makes that explicit by repeating it in verse 20.
It is not surprising that Mormon would have made this promise to Lehi even though he appears to have been unaware of the small plates record, upon which modern readers find that promise. That was so foundational to Nephite society that it was certainly passed on in numerous ways, and repeated often.