I saw a post on Reddit saying this isn’t a cryptographic key, rather it’s a hash verification code, specifically something called SHA-256.
What it does is provide authenticity of the contents of future files that are planned to be released. They can’t be edited without generating a different hash code.
Perhaps this is in response to accusations that some of the WikiLeaks reviews contain altered, fictional content. The prerelease of the hashcode would guarantee that the entire dump is as Assange released it.
A hash would only validate one document. A hash that authenticate two documents would represent a collision. Not likely at all for a 256-bit hash. I suspect it is a hash to either authenticate a future message, or is a key to unlock an archive.