On X, this means emacs-slack displays images, emoji &c. just like the web or pseudo-native clients do.
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.
But seriously, how hard would it be for a sneaky dev to make emacs with emacs-slack into a normal person application with clicky buttons and no ugly gnu? Could be worth a lot of money, or at least github stars.
Well, out of the box emacs has a clicky-button toolbar: https://i.stack.imgur.com/ml0UE.png
And using the keybindings folks expect from Windows is part of modern emacs: https://www.emacswiki.org/emacs/CuaMode
As an aside, I absolutely love the idea of every new app being written atop emacs, but I'm an incorrigible emacs user. I use emacs for email, for git, for slack and (often but not always) for web browsing. Oh, and also to edit text.