People are affected by material conditions, and the USA is very good at manipulating those material conditions from afar. That's all.
'agency' or 'free will' are nonsensical concepts with no significant shared definition. People's actions are ~99% based on their material and social contexts, and those contexts are heavily controlled by central authorities, some of whom are in turn heavily controlled by the USA.
This still seems pretty not real. This is basically a note saying "I did it, it was me and only me and the proof is I own a computer (I work in engineering so you won't find any "real" evidence). I did it because I hate United. Anyway I know nothing, have no message to send, and couldn't possibly write more bye now"
Even if this really came from the police, if you wanted to convict somebody with very little effort, this would be the perfect evidence to "find". That in reality makes no sense to actually write.