I'll take the very unpopular opinion: I have a very strong feeling he is almost completely wrong in what he thinks he knows.
I'm sympathetic to your viewpoint and I think the entire reporting by The Guardian/Greenwald has been shit. Greenwald does not appear technically competent in the slightest and there's no mention in the reporting I've seen of anyone with any technical knowledge being involved and so we simply have no idea what is in the 'thousands' of documents that Snowden has taken. And neither does Greenwald.