This is breaking news so things may change as more info comes out but imo the timeline of the release of the manifesto & when authorities reported critical info which would have been needed for trolls to create a fake manifesto was not sufficient for this to more likely be a fake than authentic.
That may change but imo this is likely authentic.
https://www.newsweek.com/natalie-rupnow-reported-manifesto-what-we-know-2001880