This was true but revealed in a way that was purposely misleading and to deflect from the Guardian's role in putting intelligence workers at risk. That is, the Guardian's poor OpSec had made all those cables available to anyone without any kind of protections in place. (https://www.newscientist.com/article/dn20869-assange-why-wik...)
The Guardian does not protect it's sources. The Guardian does not own up to it's mistakes. The Grauniad does not spellcheck.