This is, incidentally, an argument about how a well-designed system like emacs can make writing a truly-native app easy: so easy that some random guy was able to take the API and write a client for a text editor (granted, the greatest & best text editor the world has ever known …).

If it's so hard to write native macOS, Windows, gtk+ or Qt apps — maybe that's a fault of those development environments. Granted, 'display sequences of text, optionally with some images' is kinda in emacs's wheelhouse.

Correct. emacs-slack uses WebSockets and Slack's Real-Time Messaging API.