I think plenty of people knows Slack has threads. Threads in Slack are secondary and are mostly meant for having conversations that people don't care about. Zulip has a much superior threading model. Each and every conversation is a new thread and has a topic. This model makes it extremely easier to catch up and participate in conversations. Once you get used to the threading model of Zulip its hard to tolerate threading model like Slack which is really inefficient and time wasting.