Yea I think at this point it's a safe assumption that Origin will be required to play the game at all (in whatever way they wrap it). Even though Titanfall is an EAP game, you have to remember that EA literally threw money at these guys the moment they walked out of Infinity Ward. I'm guessing there's a closer relationship than the usual hands off approach you expect from an EAP game.
That said, if Respawn were wise they would have kept the rights to digital distribution for themselves (like 38 studios did for Amalur). But if they build the game to use Origin, then the best you can hope for from a possible Steam release is that it launches Origin and then the game.
Or, maybe I'm being too pessimistic.
I'd be okay with this. As I said yesterday:
I wish EA would get over Valve's DLC policy. While I don't mind using other clients/services if I have to, if Ubi can have its Steam games boot up individualised instances of Uplay, you can do the same [with Origin], EA.