It instead is a chat where a thousand group chats are open, and no once wants to read any of them.

I really wanted to like Zulip and use it as a personal chat service for a small group and it was exactly that feature that made it basically unsuitable. Forcing everything into titled threads did not make any sense for lots of user to user interactions that are ad-hoc in nature.

I didn't think it was terrible software by any stretch of the imagination - just not really suitable for informal communication.