Strawman argument. By the same argument "Manning would not hav had anything to release if the DOD had implemented compartimentalization of access to state documents." (which most modern armies were already doing before, and which the DOD now has)
Understandable, but the parent is trying to shift the blame of anyone dying to The Guardian and I would argue that blame lies on Manning by releasing hundreds of thousands of documents wholesale vs. a whistleblower like Ellsberg in where he only published data directly related to a specific crime.
Not untrue, but analogous to saying someone wouldn't have died from a gunshot if the ammunition hadn't been sold. It misses the bit where someone pulled the trigger.