The actual offensive says he knew the documents had been and “would be” obtained illegally. The key phrase is “would be”. Once he knew documents would in the future be taken illegally and agreed to receive them and publish them it entered the realm of an illegal conspiracy to obtain classified material.
This differs from for example The Pentagon Papers where the material was delivered to reporters after already having been taken. They had no foreknowledge that they would be taken.
The original indictment goes much further than that! They didn't have him on a technicality; they had him as effectively the orchestrator of the conspiracy. Who knows if that would have held up in court; I think the case wasn't all that strong on anything more than a minor role for Assange.
This differs from for example The Pentagon Papers where the material was delivered to reporters after already having been taken. They had no foreknowledge that they would be taken.