The mere existence of beliefs in the mind does not establish the veracity of those beliefs. You’d think this would be particularly obvious when the beliefs in question are diametrically opposed to each other . . . and exist in the same mind. But no, that’s exactly what “The Pro Choice Mama” is asserting here: we know that A and not A can both be true simultaneously. Why? Because they exist as incorrigible mental states, apparently.
I’m afraid this is the actual epistemology behind a lot of what passes for “liberalism” today.