Charles Ellicott Commentary Judges 11:15

"and he said unto him, Thus saith Jephthah: Israel took not away the land of Moab, nor the land of the children of Ammon," — Judges 11:15 (ASV)

Took not away the land of Moab ... —What they took was the territory of Sihon, which they had never been forbidden to take, and had indeed been forced to take by Sihon’s attack on them. It was not likely that they could enter into discussion as to the previous owners of the land.