Im curious, though. Does using a decentralized infrastructure increase the risk of the guys at the top losing control of the message?
It depends on what you mean by "losing control."
If you mean that others could take it and distribute it elsewhere, then yes.
If you mean that others could take it and CHANGE it and then redistribute, well then there are steps you can take to make sure everyone knows what the true, authenticated copy is.
For instance, let's say they publish a document and they put it on a distributed network like BitTorrent. Well, before they put it out there, they could run a hash on the original file and tell everyone what the hash is...then when people download the document, then can run the hash on it as well and compare to the one listed by Greenwald's org. If the document changed even one single bit (and I mean 'bit' in a computer sense, literally one single bit) then the hash will be significantly different.
This tactic is actually in common use all over the internet for ensuring downloads are uncorrupted.
And that's just one example. There are other methods they could use as well, but I think that's the easiest.