I can try to highlight a difference.
At least in terms of depression feeling numb and losing interest can feel involuntary; a feeling like nothing can penetrate this deep fog that you've nestle into. You sort of know you don't want to feel this way but the tricky bit is teasing away if it's "health" or "circumstance". But I think for the author this would fall under "actually unhealthy".
Contrast this with something like narcissism, psychopathy, or school shooters where the author might say: You just think they're ill because you think they're assholes, inconsiderate, or evil. They're not ill, they just don't play by your rules.
Disclaimer: Unfamiliar with the book. The preceding is speculation of the point of view as explained by the parent, but a view that I sorta resonate with.
With every school shooting or public massacre there are these long diatribes about it being a public mental health issue (or a gun issue?). Why can't they just be evil assholes who didn't have much to lose? or chose to act out in a vague revenge against society? I'm not saying I disagree but I suppose I'm skeptical it being painted as so cut and dry.