Yes, you need to import the script in your ImportJS, and follow the configuration as mentioned in the page. You cannot omit MediaWiki:Custom-Discord-id, because it's used as a sidebar for your discord server. All the other config is optional, but the first one is required.
No CSS is required at all for configuration, but yes, the script is created using both CSS and JS.
It doesn't look like you are missing something. It looks like role hoisting no longer works due to the undocumented changes in Discord's (the company) side, according to the notice on top of the page. The authors of the Discord script are working hard to restore its functionality.
Currently, any wiki who is using the Discord script will have every user listed in the user category, even if they set up the roles for individuals. Maximum users shown in the rail module is now 100.