Yeah, maybe not a boss but I thought that would be a cool change of pace. To throw something at you early instead of the usual long slow buildup. I mean it took hours to reach the first boss on VoG and KF release days, and CE only had one boss anyway. Who knows, it's all speculation of course.
It's probably a safe assumption that it's early in the raid. For TDB teasers, they showed the initial drop and some Abyss stuff like going across the bridge into the light. King's Fall I remember them showing some snippets from Totems but they also showed Golgoroth which throws a wrench in my assumptions, haha.