The argument that non-human animals do not possess some vague set of traits that make humans the only species worthy of being classified as ‘people,’

Metacognition. It's just metacognition. Not vague in the slightest. You can argue that it isn't a good criterion, but it's a very clear one.

I don't agree with the use of the word "sociopathic". It's Greek for "socially ill" and has historically been associated with pseudoscientific stereotypes applied with people with antisocial personality disorder. Mental disorders are not bourgeois, they are proletarian. They are associated with hardship and trauma, not privilege.