That's not been my experience. Typically both are developed in parallel. The integration of the story into the dev build might come later, but certain entities that enable story development should be there. Things like checkpoints, non-background characters, locations/state driven events, unique items and environments, etc... What I saw in this leak did not include anything that - it all seem generic, and that pile-o-cash loot item highlights what the mission reward system is going to be like.