Sorta kinda true. 1st parties, those who receive the files through normal means as part of the government apparatus/jobs, can't publish even/especially if it was accidentally sent/received. A random low level government office drone receiving TOP SECRET: PLANS TO NUKE CHINA on their email can't disseminate that shit without facing the government's full irritation.
2nd parties, such as wikileaks and journalists in general, have infinitely more protection. The only time you can really stop them from publishing secret information during war has been essentially defanged since Vietnam, and doubly so during the War on Terror. Even then, the government would have to actually no bullshit declare war in order to flex any of it's legal censorial powers, and even then that might not be enough considering how broad and universal 1A rights are against the government (what they were actually designed for as well).
Since the journo wasn't a government employee with certain expectations, he can't be prosecuted. Whoever added him however certainly could be.