Or HaChaim on Exodus 15:15/אור החיים על שמות ט״ו:ט״ו

1 אז נבהלו אלופי אדום, Then the chiefs of Edom were frightened. The word אז refers does not refer to what happened at this time, but to what will happen when the final redemption of the Jewish people is at hand. The same is true of the prophecy of Bileam concerning Edom being laid waste (Numbers 24,18). The same applies to the feelings of Moab and Ammon whose lands will be appropriated by Israel in that future. The reason that Ammon is not mentioned separately in the song the Israelites sang after crossing the sea, although it too was the product of Lot's daughter sleeping with him is, that it will share the general fate of the land of the Canaanites which Israel had not captured up to that time. The Torah therefore speaks of the ישבי כנען, people living in Canaan at the time though they might not have a legitimate claim on that land. The reason the Torah uses the past tense is to assure us that this future is as assured as if it had already happened.