The owner of 8Chan has already came out and claimed the manifesto was not from Crusius. And this would be easy for a webmaster to chase down in most cases. If a user posts something from one verifiable IP address location and then 5 minutes later posts from a verifiable address 2,000 miles away, they were not posted by the same source. Or if a user has always posted from one demographic area IP range and this one post is from a totally different IP area range then more than likely the sources are not the same.
Even more solid would be actually making an IP address connection with another different user on the site without any doubt of who the source truly happens to be. This would be irrefutable evidence.
Sorry, you are wrong. The owner of 8chan did not say what you are claiming. The facts are that Crusius was not a subscriber to 8chan, but posted his screed to his account on Instagram and the owner of 8chan correctly noted that a copy of the Instagram posting by Crucius was re-posted by another user to 8chan. The mainstream news media has been getting this wrong all along, probably deliberately so.
He did NOT say it wasn’t from the shooter ...he just said it was not the shooter who put it on 8chan.. He said it was posted to Instagram originally, and someone who was not the shooter found it on Instagram, then copied it from Instagram to 8chan.