Well, I look, and they have indeed been careful not to make any definitive statement on an offline-only mode, but, nevertheless, there is definitive evidence pointing against a server-based game:
1. The announcement on Rock, Paper, Shotgun where they state that "If the internet goes out for a bit, you can still play."
2. This slide from their GDC presentation, which says "Asynchronous server model: No reliance on dedicated live server running to support your play session, graceful degradation if we have server issues (highlighting by myself)