The Russians definitely found a flaw in the way the U.S. works with political information today, and exploited it for a big effect. And I don't think that flaw is fixed as we head into this election.
So, I agree with Greenwald that a broken media environment made it possible. But I disagree with him that that casts doubt on Russian involvement, or invalidates Russian activity as a topic of national concern.
Did Greenwald ever say he doubts Russian involvement? I know he on the side of "Trump didn't conspire" and feels that the rest of the media is mishandling the matter, but I don't think he's ever disputed the actual meddling.
(Correct me if I'm wrong; if he's actually disputing the existence of the meddling he'd go down a few notches in my esteem).
So, I agree with Greenwald that a broken media environment made it possible. But I disagree with him that that casts doubt on Russian involvement, or invalidates Russian activity as a topic of national concern.