I read this advice as being humble when coding as in not assuming that you got everything right the first time - in other words, test the hell out of everything

I read this advice as being humble when coding as in not assuming that you got everything right the first time - in other words, test the hell out of everything, even if you're sure it's right. Then, when you have tested it inside and out, you can be as confident face-to-face that you did get it right.