I use Sublime, own a Sublime license, etc.
I'm not talking about the colours and icons - just that it doesn't use native UI and what UI elements are implemented are quite clunky. It's very much not 'native the way Apple likes native'. It doesn't try to be an IDE so there wouldn't be much point in Apple buying it to 'compete with VS Code'.
do you think that they're not discoverable enough, maybe? How could it be improved?
Mostly, I think the Sublime developers don't consider this much of a priority and are ok with it being a clunkfest. Given that it's a small team making a living selling a niche cross-platform tool at a mere $70 a license, this is hardly unreasonable. But pretty or intuitive it ain't.
Compare this to the extension/theme management or preferences UI in VS Code. It's also very much 'non-native' but usability is clearly something the developers are concerned about.