Rashi: Refer to 2:5:2:1*.

Rashbam: We can extrapolate from it that, if the B'nei Lot received their inheritance because their father was a relative of Avraham, how much more so will you, who are his children, receive yours. ¹

Rashi #1 and Rashbam: The Zamzumim, who originally inhabited the land of Amon, were called 'Refa'im', just like the Eimim (whose land Mo'av took over), only this is not Eretz Refa'im ¹ that Hashem gave to Avraham (Bereishis 15:20) who originally inhabited the land of Amon, were called 'Refa'im', just like the Eimim (whose land Mo'av took over). In fact, they are synonymous with the Refa'im whose land Hashem gave to Avraham, ² only the Mo'avim and the Amonim changed their names on account of their stupendous strength. ³

Rashi #2 (in Chulin, 60b): To disillusion Yisrael, who, thinking that this referred to the Refa'im that Hashem promised to Avraham, therefore had in mind to attack them. The Torah is therefore informing them that they are not the same Refa'im, and that they are prohibited from fighting them.

Ramban, Targum Onkelos and Targum Yonasan: The word means 'conspirators' ¹ - because whatever they planned to do they accomplished successfully.

Hadar Zekenim: Because t hey were Chachamim and war strategists ('Ba'alei Mezimos). ²