It goes over a utility called "suc" (Simple Unix Chat) that implements server functionalities from Slack, Discord, etc. using a very small codebase.
The novel part is it leverages existing unix tools and methodology instead of re-inventing them.
- Auth is handled by SSH.
- Channels are just a file and admin/mod controls are handled by user groups and file permissions.
- Support for rich text, file uploads, etc. aren't a concern as you just write whatever data you want to the channels and let the client interpret it.
- Bots are very easy as you just pipe to/from the channel files.
You're not going to be making a perfect slack clone with the 5 lines that write messages to the channels from the article, but I'm pretty impressed how far you can with really simple unix tools.