It’s a question of internal consistency. The video you linked skipped that by starting with actual actors, but “generate a 30 second video” means no shortcuts.

There’s a common issue in AI generated still art for dynamic shots like someone dunking a basketball to have extra arms. This doesn’t show up in pictures of people standing still because the training set is completely dominated by people standing with their arms to the sides. But in dynamic shots either seems fine and it doesn’t understand the underlying limitations the way a human does.

I don’t think it quite needs AGI to “make a 30 second clip of a fireball fight” but it’s much much harder than a still image of a “wizard holding a fireball.”