Very little. A couple of graphs, maybe. It is, as you say, a well presented rehash of his previous interviews and writing, and I don't think there's anything wrong with that. The only thing that's wrong is to call it a documentary, which it clearly isn't -- at least, not in the ordinary sense of the term.

Until you pointed it out, I wasn't aware that Chomsky didn't write it himself. I would ordinarily find that troubling, but the script is closely based on Chomsky's previous writing. I'm confident Chomsky would have checked it carefully and would not have agreed to participate if there was anything he was unhappy about.