Like, let me share more of my reasoning:
Look at all the articles that were made about the shooter. They all mention 'online communities' but don't share any more information besides that.
Literally the only source that claims he came from Blockland was TheDailyBeast and other news sources which directly cite TheDailyBeast.
I think the reason why sites like CNN, FOX, and MSNBC don't say anything more than 'online communities' is because that's all people actually know. To get the full, nitty-gritty details of a federal investigation, you have to submit a Freedom of Information Act (FOIA) request, which takes literally weeks to process. The reason why all the mainstream news sources are being vague is because they're only operating off of cursory, summary details given to them by authorities.
More than likely, here's what happened: the journalist who wrote that article for TheDailyBeast managed to find William Atchison's page on Encyclopedia Dramatica, which shows up pretty high in the non-news google searches for his name. Then, they noticed that it mentions in the article that he was allegedly a member here. That journalist, realizing that he now has some piece of information that nobody else has, decides to come here, reads this thread, and then wrote an article based on it.
But does that mean Sephiroth is the shooter? No, it means a journalist read this thread.
This thread and one post made by TomTheGeek are currently all of the evidence that this guy had anything to do with Blockland whatsoever.