By default Discourse supports categories and sub-categories. If you use sub-sub-categories, that’s a custom setup that you enabled on the server console.
Right now the component doesn’t check for nested categories beyond the sub-category level. So it’s a limitation of the component. If you want to add support for further nesting of categories, you could ask the maintainer if that’s an option they’d consider as a PR. However, as this is targeting a non-default Discourse setup you might need to fork the component to include the feature.