What does Judges 11:19 mean?

Jephthah is explaining that the king of Ammon is wrong in claiming Gilead belonged to his people before being stolen by the Israelites. For one thing, Israel went to great pains to avoid even stepping foot in lands belonging to other peoples (Numbers 20:14–21) on their way across the Jordan River into the land of Canaan (Judges 11:12–18). When they did arrive in territory of Gilead, the Ammonites were not yet there. They were farther east. Further, the Israelites did not even want to stay in Gilead. They just wanted to cross over the Jordan and move on. To do so, though, they would have to travel through the region that became known and Gilead, and it was occupied by the Amorites—not the Ammonites.

Since the Israelites were still not interested in war on the east side of the Jordan, they sent messengers to the king of the Amorites and king of Heshbon. They requested to travel through those territories to get to their own land. The overall point of Jephthah's history lesson to the king of Ammon is that Ammonites were not involved in any of this.