Assange allegedly attempted to help Manning do this using a username which was not hers in an effort to cover her virtual tracks. In other words, he allegedly instructed her to hack the Pentagon, and offered to help. This is not an undertaking any working journalist should attempt without knowing that the immediate consequence will be the loss of his job, his reputation, and his freedom at the hands of the FBI.
I might further direct you to Assange’s own unique brand of journalism, when he could still be said to be practicing it. Releasing U.S. diplomatic communiques which named foreigners living in conflict zones or authoritarian states and liaising with American officials was always going to require thorough vetting and redaction, lest those foreigners be put in harm’s way. Assange did not care—he wanted their names published, according to Luke Harding and David Leigh in WikiLeaks: Inside Julian Assange’s War on Secrecy. As they recount the story, when Guardianjournalists working with WikiLeaks to disseminate its tranche of U.S. secrets tried to explain to Assange why it was morally reprehensible to publish the names of Afghans working with American troops, Assange replied: “Well, they’re informants. So, if they get killed, they’ve got it coming to them. They deserve it.” (Assange denied the account; the names, in the end, were not published.)