Well, no, not exactly. The Guardian published the password. Wikileaks failed to secure the encrypted payload. They both had to fail for the security breach to have happened. Irresponsibility is shared there, and as best I can tell, Julian is embarrassed and attempting to salvage ego with a dumb "I meant to do that" sort of maneuver.
The Guardian is being a bit silly in complaining now, after the data is already out there - anyone with an interest has already found a torrent.
But really, the whole thing is silly, given that the cables were available very widely to (as I understand it) millions of US folks already. I simply don't believe that documents shared with 7 figures of people, security cleared or no, don't find their way to people who have an interest in such things.
Most of the hot air being puffed about this has to do with what is public-public, instead of private-public. It makes a difference. (To pick a different example: "everybody knows" that many cops in the US arrest routinely people who annoy them on bullshit charges. This is private-public knowledge. Now imagine documents hypothetical leaking about this being policy. That would make it public-public.