I'm not clear on why people keep saying the manifesto might not be real. I haven't read it, only seen various excerpts posted here and there, but my impression is that it's a scattershot, ideologically incoherent mess that boils down to "I have no real principles, but boy do I hate black people", and it's heavily plagiarized from the works of other mass shooters as well.
Which sounds exactly like what I'd expect from manifesto written by some terminally online idiot zoomer barely out of high school?