It's not hard to get a UI like Slack's. The actual chat window is just like what IM clients have used for decades. One side has the contact and chat room (channel) list combined instead of in a separate window. Like in IRC instead of XMPP and other chat systems. Put the settings and away management in a header or footer and you've got slack.

It's not like it's revolutionary. They put two and two together in the only reasonable manner for a single-page application.