I would think it's not even a question at this point.
There's still that blurred out mode that they would not even show us as well.
It's called "Spirits", by the way.
Edit: I guess the trend of characters dying in the trailers isn't just for show.
We don't know the full name yet, but I think it might be kind of a mixture between Classic/Adventure/Story modes
And to answer the OP, there was not much to question in it's authenticity as a brand new Smash game. It was aanounced as Smash 5, then it is.