On a surface level, yes many of them actually do believe their own B.S.

But leftism like any religion or cult uses a series of thought prevention tools to stop people from being able to question these beliefs fully.

People who are brought up left wing are often sufficiently indoctrinated that even when presented with absolute proof or evidence that their beliefs make no sense, will likely refuse to acknowledge it and will double down and/or dismiss the evidence as heretical (bigoted, racist, transphobic etc.).