Zulip is more of a Slack-like instant chat system with threading as a first class citizen; CQ2 looks like threads only exist in the context of one "root" document vs a channel in zulip where threads can intermingle.

And neither Slack nor Zulip are suitable for complex discussions.

Zulip's core design goal is to make it effective for complex discussions, and our users tell us that it's working. E.g., cutting-edge math research https://zulip.com/case-studies/lean/; Rust language development https://zulip.com/case-studies/rust/.