> The mental model might take an hour or two to click, but you can think of it as Slack rooms where the only thing you can do is open new threads (can't post messages in the room itself, only in the threads).

I've used both Slack and Zulip, and I wouldn't describe it that way. Slack threads feel more "isolated" by default, hidden away from the conversation, and you have to go look at each one; a Slack channel where you can only create threads doesn't feel like it'd be as appealing a UI as Zulip. On Zulip, you can look at the entire stream to see messages from different threads in real-time, or you can look at just one thread and see only the messages from that thread.