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It's saying that it's easier to see flaws in other people than in ourselves.

Should be “Others’”

But it's "others" (no "'") in the link ;)

Either way, it's not a universal truth. Some of us work hard on this (probably because we came across Confucius in our youth).

Isn't the fact that it needed hard work an indication that the statement is useful for the average person?

Useful for the average person, absolutely. I don't think I understood the question.

Should have been "behind our eyes" i.e. ourselves. Behind our back is empty space with other people in it. Too ambiguous/poorly thought out.

I think behind our back is a metaphor for "well hidden, even from ourselves"

The only context I've heard is "talking about someone / doing something behind their back." Yeah I suppose you can construct more meanings for it, as OP authors apparently wanted to do. And down the BS rabbit hole we go.