Ken Klippenstein is a very reliable journalist and this version of the manifesto contains the snippets that have been released by law enforcement. Also, considering the thing was hand-written, that very long version involving his mom is dubious. (And there’s not any good evidence that his mom is in anything besides decent/good health)
I'd like to add that he hasn't been found guilty for anything, for all we know he could be a completely innocent person who just happened to fit the profile of someone they could pin it on.
A lot of the discourse seems to revolve around why he did it (e.g. from the article: "nothing he read or posted explains why he gunned down an insurance executive better than this single image in the background of his Twitter profile") but it's important to remember that there's a reason we have presumption of innocence, especially when it comes to how many innocent people have spent their life behind bars or even gotten executed.
It can be discussed without the assumption that he is guilty. I think it is dangerous for the public to remove the presumption of innocence, especially in cases like this where the ones affected are the ones with power.
I would even argue that it's more important to argue with the presumption of innocence. Someone killed a health insurance company CEO and a lot of people are rejoicing over it, then someone who is quite possibly innocent and who seems to have suffered from health insurance company decisions has been arrested for it. How many other people have experienced something similar to Luigi? How many other people fit the profile because the system destroys that many lives? He has already suffered, how much more will he have to suffer for something that he possibly had no connection to, for the benefit of the people who seem to have already ruined his life?