This is correct. The core value-add of e.g. Slack is not actually "transmit messages between authenticated users." There were dozens of other applications, both commercial and open source, that already did exactly that when Slack came out and killed them all off.

The core value proposition of Slack is next-level beyond that: it adds a visually appealing, highly intuitive UI/UX that almost anyone can sit down and immediately use, without any studying or setup or manuals or training. No technical skills are required. You don't need to understand anything beyond "type username and password"; the rest is self-explanatory.