You think they're all judging you. They're not. They're too busy hoping you're not judging them to hold an opinion at all.

The thought behind it

This line works by turning the expected explanation inside out: the thing you were blaming turns out not to be the thing. The Honest Mirror trades reassurance for accuracy, on the theory that accuracy is the kinder of the two. Everything hinges on “opinion”, which is the part a politer version of this sentence would have left out.