Australia has pretty brutal anti-defamation laws. Just look at what YouTuber Jordan Shanks aka FriendlyJordies went though. A politician basically admitted to corruption, Jordan essentially quoted him in the video, but then Jordan was found to be guilty of defamation because what the politician said was covered by parliamentary privilege. So, basically, even an absolute truth is not always a defence.
That being said, the dude in question is long dead, so I'm not sure who exactly could claim to have been damaged by the assertion that the dude in question was a traitor.