Sforno on Numbers 21:31/ספורנו על במדבר כ״א:ל״א

1וישב ישראל בארץ האמורי, from all that has been reported here it is clear that the Israelites at that time did not settle in territory under the rule of the Moabites. This is precisely the point made by Yiphtach in Judges 11,15 when he rejected the claims of the Moabites at the time he was judge, saying that Israel had never taken land belonging to either Ammon or Moav