These verses, like Deuteronomy 2:10-12, are in all likelihood an addition made by a later reviser.

Deuteronomy 2:20

Zamzummims - A giant race usually identified with the Zuzims of Genesis 14:5.

That also was accounted a land of giants - That was accounted the land or territory of the Rephaim.

Zamzummims - Supposed to be the same as the Zuzim, Genesis 14:5. Of these ancient people we know very little; they were probably inconsiderable tribes or clans, "pursuing and pursued, each other's prey," till at length a stronger totally destroyed or subdued them, and their name became either extinct or absorbed in that of their conquerors. From the 10th to the 12th, and from the 20th to the 23d verse inclusive (Deuteronomy 2:10-12, Deuteronomy 2:20-23), we have certain historical remarks introduced which do not seem to have been made by Moses, but rather by Joshua or Ezra. By the introduction of these verses the thread of the narrative suffers considerable interruption. Dr. Kennicott considers both these passages to be interpolations. That they could not have made a part of the speech of Moses originally, needs little proof.

That also was accounted a land of giants,.... Ammon was so reckoned as well as Moab, Deuteronomy 2:10.

giants dwelt therein in old time; the Rephaim dwelt there, as they did also in Ashteroth Karnaim, Genesis 14:5.

and the Ammonites call them Zamzummims; they are thought to be the same with the Zuzims in Genesis 14:5 who had their name, as Hillerus (c) thinks, from Mezuzah, a door post, from their tall stature, being as high as one; and for a like reason Saph the giant might have his name, 2 Samuel 21:18. The word Zamzummims, according to him (d), signifies contrivers of evil and terrible things; they were inventors of wickedness, crafty and subtle in forming wicked and mischievous designs, which struck terror into people, and made them formidable to them.

(c) Onomastic. Sacr. p. 158, 288, 289. (d) Onomastic. Sacr. p. 161, 310, 428.

(That also was accounted a land {i} of giants: giants dwelt therein in old time; and the Ammonites call them Zamzummims;

(i) Who called themselves Rephaims: that is, preservers, or physicians to heal and reform vices: but were indeed Zamzummims, that is, wicked and abominable.

Zamzummims: also called, Zuzims

2:8-23 We have the origin of the Moabites, Edomites, and Ammonites. Moses also gives an instance older than any of these; the Caphtorims drove the Avims out of their country. These revolutions show what uncertain things wordly possessions are. It was so of old, and ever will be so. Families decline, and from them estates are transferred to families that increase; so little continuance is there in these things. This is recorded to encourage the children of Israel. If the providence of God has done this for Moabites and Ammonites, much more would his promise do it for Israel, his peculiar people. Cautions are given not to meddle with Moabites and Ammonites. Even wicked men must not be wronged. God gives and preserves outward blessings to wicked men; these are not the best things, he has better in store for his own children.