Here is the thing. If you carefully watch the video again he says comment that "he can show up as a boss". If it were a story mode wouls they make that comment?
I'm not saying it can't be a stage, and I also don't believe that 103 or 100 is the max # of levels we will get.
I'm just saying that as of right now, I'm just going to assume that "stage" is nothing more than a story stage until told otherwise.
The game is large enough with all of its characters and stages that whether or not we get anymore is irrelevant to me. More is always good, but I'm not going to complain if that ends up not being a playable stage.
I believe it will probably be a stage, but in the same sense the biggest argument against it is staring at us in the face. We had a huge clear view of the stage being showcased. The dragon flew around, and fought people on it for quite some time. It wasn't like a quick flash. Yet despite how anyone with half a brain could see that is not a stage we have seen before, he did not include it in his 103 stage count.
Also he didn't say OVER 103 stages, or Over 100 stages. He just said the number. With songs, we got over or roughly. With characters, no amount has been stated. Stages is like the only concrete number we have gotten. Does that mean that it is the final number? Maybe, maybe not. For now, I'm going to assume yes that is final.
Edit: BTW, anyone know how many stages are missing that have been in past games?