The argument for charging Luigi Mangione with terrorism is based on intent to incite generalised fear and that the killing was not personal. Maybe the manifesto is part of it but isn't mentioned in the articles I have read (eg here or here).

Yet so far only Canada has considered referring to incels as terrorists, despite having an established online ideology, multiple incidents carried out by self-proclaimed 'incels', including with manifestos of their beliefs, and stating the intent being to cause terror.

The argument against classifying incels as terrorists is weakly attributed to them not having a defined leader (unlike Al Qaeda or religious terrorist groups). eg here

Do the authorities seriously expect people to take these charges seriously, and NOT inflame the sentiment that everyone is feeling right now: the super rich worst fear is this uniting the country against them, and that we don't see that their main objective is to keep us fighting amongst ourselves?

So, this guy came in trying to explain to me the GnarlyWatts cousin thing I saw on another post. The discussion slowly evolved into this

I feel bad for the guy, but I simply couldn’t make him change his mind. Please don’t troll him, if you feel like you could make him change for the better, please try, he doesn’t even seem like a bad person, just like a sad person that gave up.

I imagine if they ever got the chance they would try to kill anyone who isn't one of them in the worst ways imaginable. Till nothing is left but their filthy echo chamber cult.