That 's liberalism as defined by the Catholic Church. Matthew is talking about political liberalism, notice he talks about conservatives.
In the end, only the Catholic definition is the important one, for all liberals (ALL Protestants, Jєωs, Muslims, Heretics and schismatics, pagans....) go to hell, whether they are conservative or not.
Of course, I understand what he is talking about and as I said, I entirely agree with him. Today's liberal parents, are the result of yesterday's Catholics. Today's liberal parents are two generations, (going on three) from the Woodstock generation - a time in history when liberalism went wild - within the Church, and then, of course, society. "The sins of the parents.." etc,.
Mental illness is one thing, evil is another - and today's society's mental illness is rooted in evil, the evil of liberalism, which is a sin. Hence the result of the sin of liberalism, is what we call the mental illness we see in today's society. What we see today is evil, it is sin. The lying, "fake news", propaganda, all the injustices and etc., are all evils, they are sins - which we are calling mental illness because it happens on such a large scale.
The sin of liberalism is like some type of addiction or disease, it's progression makes those infected not merely mentally ill, not merely ignorant, it makes them glory in their ignorance. And that's what we are seeing today.
I think instead of merely reducing it to a political position by calling it "Liberalism", it might be better to remember what it actually is, namely, "the sin of Liberalism", which helps explain why it's adherents actually are mentally ill.