Look at section 9.04 and see if that helps.

9.04 .... What are the facts of your case?

Never had that one, but there is a case in the TDCAA code book on page 22 in which self-defense was raised as a defense to criminal mischief, where the "criminal mischief" arose out of a use of force against another, so I think under the right facts it could apply to terroristic threat also - depends on what he's alleging the victim did first (e.g., V pulled his gun so D pulled his, then V called cops). Good luck!

Well, what it sounds like is they charged him for threatening to kill someone, and he is saying I threatened him in self defense. Section 9.04 deals with threats of force rather than use of force which is regular self defense.