Founder of Struct here. I looked at Zulip before starting Struct. And I'm sorry -- I don't think it's the same. Threads in Zulip are really sub-channels, and the idea of a unified feed like "All Threads" doesn't exist -- at least, that was my perception of Zulip. And just looking at the site right now, it feels the same as before. I could be wrong.
Struct is different. It's a reimagination of what a chat platform would look like if you were to completely give up the idea of chats in a serial log of channels (IRC, Slack, Discord), and embrace threads and feeds whole-heartedly. When everything is a thread, the platform can work remarkably well for users.