"In February 2011 David Leigh of The Guardian published the encryption passphrase in a book;[6] he had received it from Assange so he could access a copy of the Cablegate file, and believed the passphrase was a temporary one, unique to that file"

Assuming David Leigh was not lying, Assange should have been more clear with the security implications. (then again, I see no reason to publish the temporary key in the first place). Still at that time it was not not known, except for maybe some intelligence organisations. So if really concerned, one could have done many different things to protect informants, delay the time, instead of publishing it officially for the whole world to see.