It doesn't matter what you think is investigative journalism.
What Assange did is investigative journalism... as proven by Bellingcat, that literally bought private phone, flight, bank and train ticket data on private individuals to show how FSB tried to kill Navalny.
well bellingcat went (and still goes) looking for information, sources and then publishes article on those stories, which is not exactly what Assange did, which was mostly making confidential documents (sent by a variety of sources) available to the wider public
Assange first provided the cables to newspapers like The Guardian, Der Spiegel and El Pais, so that they could write articles about them. After they had had time to work through the material and write up a series of articles, WikiLeaks published redacted versions of the cables.
