Earlier quoted context omitted.
It says pre-commitment so it looks like a dead man switch. If something really bad happens to Assange and someone releases documents about Kerry/Ecuador/FCO we can look at the hashes and see if they match. That way we know it's not some fake leak by an opportunist.
If that is assassination-insurance should those bytestrings be keys to decrypt files that are already in the wild? Anyhow, I find the tweets very interesting. But I lack the context to know what they mean. Were those tweeted before the blackout? After? The UK FOC thing is particularly interesting. This might sound silly, but perhaps somebody at MI5 was not happy the possibility of Foreign Office correspondence being…
They are not the keys because nothing that terrible happened yet. All they say is that they have files on those topics, and if someone in the future releases keys to some files, and those decrypted files' signature is as it says on the tweet, then they must come from Wikileaks, since they posted that signature.