Does not match my personal experience over several months of daily use. Zulip is designed by and for software engineers—which is not a compliment. The whole experience feels like opening vi for the first time. Nothing is obvious… not even writing a message.
The threading model is probably how all other chat apps should work, but I don't think that the ui/ux that actually exists in Zulip is good.
Clicking on a message is how you open compose to reply.
Also, there are giant buttons at the bottom of the screen to compose a new one.
Yes, you do need to learn a bit about how to use the tool, but that's true for any new tool.
It's very well designed from the ground up and gets all the little details right (real name + mail address handling, not mangling pasted content, full text search, hell, they even ship with Nagios checks for their backend services...).
The threading model is second to none - once you've become used to it, using any room-based chat software feels cumbersome and "wrong".