I never saw him as a warmonger to begin with, evidence was very contrary to him being one. People ignored that evidence and made him out to be one previously, and will continue to do so regardless. Because they hate Genn (and probably love Sylvanas and think she can do no wrong).
It's not like this is going to suddenly change people's minds, whatever it is, especially if it's revealed in a novel that a very small amount of the playerbase will end up reading. Blizzard could probably outright state that Genn is a great guy and not a Warmonger or whatever and people would still call him one. Logical reasoning was never really relevant to people hating Genn.
In before "Sylvanas didn't murder Genn's son" "Sylvanas was forced to invade Gilneas" "Sylvanas attacking Gilneas was nothing personal" "Genn started the war by attacking Sylvanas in Stormheim", etc. I'm sure I missed a few, please bear with me.