Earlier quoted context omitted.
It is a cloistered mind that cannot make that leap.
Cloistered by the filter bubbles. Though Twain's essay suggests it's not a new phenomenon. People tend to group together with those who believe similar things, each confirming preconceptions of the others. From inside of such a group it always looks like everyone else are sheeple that need to wake up.
Are you saying that people are not aware that their opinions may not be as ironclad as they sometimes seem?