Deuteronomy 2:20

That also was accounted a land of giants; in old time the giants dwelt in it, and the Ammonites called them Zimthanee,

The word Zamzummim is inflected according to the rules of the Hebrew language into which it has been assimilated. Compare, “the satraps” [Esther 8: 9].

TEICHASHEIV’ (ACCOUNTED). The land of Ammon, too, is considered part of the land of Rephaim; in other words, it is included within it. Similarly, from the Shihor, which is before Egypt, even unto the border of Ekron northward ‘teichasheiv’ (is accounted) to the Canaanites. So also, the sons of Rimmon the Beerothite, of the children of Benjamin; for Beeroth also ‘teichasheiv’ (is accounted) to Benjamin. It is possible that such is also the meaning of the expression written above [with reference to Moab], these also are ‘yeichashvu’ Rephaim, Scripture stating that the Emim who formerly dwelled there, Verse 10. are also “accounted” as an expression of importance and esteem. Here Ramban explains it as “accounted” [to the Rephaim]. Rephaim since they were part of them, and they were as the Anakim, and therefore they [the Moabites] called them ‘Emim’ [because their “dread,” like that of the Anakim (giants) lay upon the people]. This is the correct interpretation.
Thus the land of Rephaim was very large, and from it, Moab and Ammon took their lands. The balance remained for the Rephaim themselves, for Og, who dwelled in Ashtaroth, was of them, as is stated, and they smote the Rephaim in Ashteroth-karnaim., Verse 5. And in the Book of Joshua it is written, and cut down a space for thyself there in the land of the Perizzites and of the Rephaim. Therefore, Scripture says of the border of Moab and Ammon that it is “accounted to the Rephaim,” [still calling them “Rephaim,” the name by which they were originally known]. The Moabites and the Ammonites were the ones that changed [the name of the Rephaim] to other names: [the Moabites calling them] Emim and [the Ammonites calling them] Zamzummim.

ארץ רפאים תחשב גם הוא, as the land of the Refaim which G’d had promised to Avraham at the covenant between the pieces. (Genesis 15,20) Even though these Refaim had been dispossessed; if the descendants of Lot could dispossess the Refaim of old, it would certainly not be a problem for the Jewish people to dispossess people who had come by this land not by ancestral right but by conquest.

תחשב, although it is considered as “the land of the Refaim, it is not officially called thus, but is part of the Kingdom of Og the king of Bashan. This is spelled out in detail in 3,13. In that verse the Refaim referred to are the ones mentioned by G’d to Avraham in Genesis 15. This is why the Israelites inherited these lands.

ארץ רפאים תחשב אף היא, there can be no question that the Ammonites had no legal right to dispossess these people; the only reason why they succeeded in doing so was because it was G’d’s will.