Another point: any intelligence agency would not let it be known that they hacked a server because it means that source be no longer be available, either because they correct the security flaw, or the target abandons that server all together. Also revealing the hack reveals capabilities, methodologies, etc.
That really doesn't apply here, because the server is long gone.
And they aren't revealing anything about capabilities or methodologies: the OS and software had many known compromises. Any decent security researcher would have been able to roam around at will.