That seems silly to me. Just because someone that has chickenpox hasn't been formally diagnosed as having chickenpox, that doesn't mean they don't actually have chickenpox.

Maybe none of these folks are truly suffering from mental illness, but what would that mean? That they're untreatable? That can't be right. So then what are they afflicted with?

It seems like they're trying to draw a line between clinical "serious" mental illness like schizophrenia and what's happening with mass shooters ... but I don't think folks that are saying "mass shooters are mentally ill" mean things like schizophrenia.