Well... no...
First of all, the Ammonites were part of the group that fought with Eglon, King of the Moabites. So, the Ammonites had already been defeated in chapter 3. This is not the first time we've seen them.
Secondly, the Jephthah cycle runs from 10:6-12:7 and the beginning of it 10:6-18 shows no signs of a "flashback," or any kind of out-of-order reporting of events. In fact, it would seem the use of the wayyiqtol is indeed outlining a chronological sequence. That isn't so obvious in English translations, but it is evident in the Hebrew.
Third--and back to the person of Jephthah--if you are holding him up as a good example (arguing he didn't kill his daughter when the text says he did), how do you process his subsequent actions, namely his killing of fellow Israelites from Ephraim?
Jephthah is not a good man, and he isn't presented that way by the author, but you'd have to know the flow of Judges well to know that.
The Archangel