Can you prove he didn’t write it?
Nut bars can be quite good at that kind of thing!
For example Teddy K the Unibomber!
Anarchists from the mid to late 1800s often left manifestos explaining why the deed had to be done for “the people”!
There’s enough such manifesto’s out there from history he also could have just copied one, copied one with some changes & “improvements” here and there to make it “his”.
Easy peasy go to the internet, go to the library. I am sure he chose internet. Library would be too much work.
Nut bars don’t always just sit in the corner and drool, they can be quite cunning & clever.