That recommendation is not about a root to enter a graph or related resources. Typically a REST API implementing HATEOAS has many potential entry points. E.g. for a blog entry points could be the posts, the authors, or tags. I would recommend documenting the supported entry points.
The main difference between documented entry points and other URLs of your API is only the stability guarantees you are providing for the URLs. URLs only discoverable over linkage within a JSON:API document may change at any time. While changing those URLs documented as entry points would be a breaking change. And covered by change management processes.