Bruce still didn't say everything that was on his mind. In fact he had a lot of trust in Zach but didn't want Diana to be angrier than she already was after finding out that her brother had become a lot like those fanatics. Inwardly Bruce felt that Zach would never take over his position because he didn't seem like such a person who would backstab his friends.
Thanks to Bruce's timely fire fighting and apt situational reading, Diana's earlier anger vanished as if it had never been there in the first place. What replaced it was just frustration; pure unadulterated frustration that made her want to pull her hair out. It wasn't that she was against Zach in any way. She too felt grateful to him but the fact that their family had started to seem like his from some time ago made her somewhat unsettled and even slightly agitated.
