I'm guessing Berserk is hard coded. Double Grip is simply not one of Knight's innate abilities. Per definition, it is not an innate ability even though there is a bit for it. As far as I know, there are no more tables to define innate abilities.
BUT we cant use Barehanded without equipping it
What are you saying here? The Monk has the "Innate Ability: Barehanded" bit set. When you master Monk, Barehanded transfers to Freelancer/Mimic and improves their Fist damage without needing to equip Barehanded. How is it any different from Counter-Attack?

Knights can hold things in both hands!
Hic!
This piece of dialogue suggests that Double Grip may have been intended as an innate ability for Knight during development of the game. They may have decided to remove it because it was too powerful.