What separates the goats from the sheep is that goats don't blindly follow.
So the dealer looks in his little mirror and sees either the card that makes a BJ,or nothing.You don't need to study his body language to find that out.
You're right that dealer tells are insignifcant in peek games. But they must be referring to games where the dealer actually looks at the card. I'm sure there are still games like this out there somewhere...