The Guardian is actually involved in the case: the "Guardian’s 2011 WikiLeaks book" contained the full password of the encrypted file containing the unredacted documents and that encrypted file was already (it is argued, not per the instructions of Wikileaks) mirrored to different locations.
The Guardian of course denies its own responsibility, because if they wouldn't, it would mean that it was the Guardian who de facto "published" the files by publishing the password to the file, and not Assange, and it would be them fighting the extradition now.
There's no way that the prosecution in this case would go after the Guardian at this point in time.
Remember, the whole playbook here is to target the weakest possible link, i.e. an individual not associated to a traditional publisher, who is being attacked personally (i.e., casting doubts on his mental health after ensuring that he was forced for years to live under circumstances that would erode anybody's mental health).
They are doing this against a chosen target because they hope to establish precedent which can then later on be used against more difficult targets.
The encryption key was to the archive that Assange shared with the Guardian journalist working on the story, David Leigh.
The encrypted archive was apparently mirrored on several sites, meaning that when Leigh published the key, anyone could access the full archive of unredacted cables. I guess Assange could be faulted for working with traditional journalists who aren't so competent with technology. That would be an ironic reversal of the usual complaints about WikiLeaks. But at the very least, Leigh should have known to at least ask Assange before publishing the encryption key.
The Guardian turned around and blamed WikiLeaks, throwing it under the bus for the Guardian's own screw-up.
The Guardian of course denies its own responsibility, because if they wouldn't, it would mean that it was the Guardian who de facto "published" the files by publishing the password to the file, and not Assange, and it would be them fighting the extradition now.