>> Then when you are told about it, you’d make up excuses trying to explain
  >> why it is totally normal.
  > That's a pretty incredible rhetorical device. "If you
  > disagree, you're fundamentally incapable of reasoned 
  > thought."

No, that's not what Zed's saying at all. He's saying that people often respond to valid criticism of things they know well by justifying the behavior being criticised.

Ie, people mold their way of thinking to suit their existing tools. Since they're familiar with how things are, they have problems seeing what could be.

It's not an ad-hominem attack at all. Think of the academic who has no idea of how to teach because he knows his subject so well he can no longer see it from the perspective of an outsider.