Either Greenwald is correct, or he isn't. His opinion on non-factual things should have no bearing on that.

He isn't correct.

One's opinion has a very profound bearing on how he interprets and report on facts.

Facts should be independently verifiable.

Verify away. But facts are nothing more than isolated points. In my reading of Greenwald, he (usually) selects the facts that push his narrative. So verifying his facts is meaningless in the context of establishing the veracity of his broader claims. You need more facts than what he presents to see the full picture. And then his narrative usually falls appart.