Um, actually no. They've already announced that you don't choose characters really going into to the 1P mode. For example: In the begining you choose either Mario or Kirby and fight against whoever you didn't choose. Then, as that person, you fight Petey Pirhanna and save, by breaking the container, either Peach of Zelda. Then you're a 2 person team fighting as you get blown off of the Halberd. Then, you're Pit, who later meets up with whoever you DIDN'T pick in the very beginning. You have limited choices in the beginning but later and later in you get to build up you're 2-player teams differently every time. You can choose to be Pit or, say, Kirby if you picked Mario originally! And the second player is helping you. If it's a friend, they help you, and if they fall behind they get 'sped up' to the screen.
Also yeah we have seen this video plus more content from that level.