Well, for one thing, the executives in charge at Cartoon Network are cartoon fans. I mean, these are people who grew up loving animation and loving cartoons, and the only difference between them and me is they don't know how to draw.
Yes, it's a prequel. It tells the story about how the girls were born with superpowers, but they weren't necessarily heroes at the beginning of this movie, so the movie is about the events that happen in their life to make them decide to be heroes.
There's a lot you can do without words.
The storyboard artists job is to plan out shot for shot the whole show, write all the dialog, and decide the mood, action, jokes, pacing, etc of every scene.
Anything you see on the screen was at one point approved by me.