Yes, Traiel is correct. A shield that takes equal to or greater DAMAGE than its hardness gets a dent. Since the shield's hardness reduces the damage, a shield with a hardness of 5 would have to block an attack of 10 to get a dent.
Jimmy the fighter blocks an attack of 11 damage.
The shield's hardness is 5.
Jimmy takes 6 damage.
The shield takes 6 damage and receives a dent.
Jimmy the fighter blocks an attack of 9 damage.
The shield's hardness is 5.
Jimmy takes 4 damage.
The shield does NOT receive a dent.