Or that GCHQ does do it, but because they're GCHQ and not NSA that it's irrelevant?
Because now you have evidence that GCHQ does it you should allow the possibility that NSA does it. The reason you don't have evidence that NSA does it might be because NSA is a secret organistion.
Snowden did not gather everything. Maybe he just missed it? Or maybe GCHQ does it but NSA doesn't? We know that other bits of the US government have different levels of online presence so I'm not sure why you're so hostile to the idea that NSA has people that disrupt online conversation about NSA.
I for one try to only accuse folks of doing things I actually have evidence of them doing, and I know this sounds crazy, but there's currently no evidence the NSA is on HN astroturfing comments, so maybe we shouldn't pretend like we know things we don't.