Hmm... not necessarily. Based off what you just mentioned, I'm thinking this is how the game will start off :
- Noctis & co. at Lucis while treaty talks are going on
- 2nd last day of treaty talks, to conclude next day.
- Party on the night before the treaty signing, meet Stella scene.
- Nifl attempts a night-time assault at Lucis
- Regis dies/or escapes with Cor
- Noctis escapes with his bros
- Enter Duscae the next morning
- Radio station still unaware that Lucis has been taken over by Nif
- Thus talking as if the treaty will conclude that night, even though they already got back-stabbed
- Nif sending patrol ships to find Noctis
Don't Noctis and Co talk about heading home? Also by their mannerism, especially's Noctis, it seems like nothing has gone down.