I imagine JetBrains isn't too concerned with mobile interfaces considering desktop software is their bread and butter

That would be a laughable lazy decision if it is so in 2022 not to care for mobile usability.

They have AppCode and Android Studio targeting mobile as well.

Even then, YouTrack (at least the one for JetBrains products) is extremely slow and annoying to use

They have Jetbrains Space and Youtrack as mobile apps.

This is not a general purpose responsive web UI toolkit. It's a toolkit for their IDE products. Nobody is using those on mobile.

Well, not for IDEs, but web-based developer tools.

The linked webpage is generated with Storybook.js, a generic web component display framework, and is not itself en exemplar of the showcased UI library.

I really hate when a dev makes excuse blaming an external library for lack of integration. It is them that chose whatever the library and didn't care to fix.

Why would JetBrains care, at all, how this site looks on mobile? It's made for developers to prototype components.