Taking the familiar example of a blog post/article, would it be possible to return both the comments on the articleand the authors of those comments?

The following quote (emphasis mine) leads me to believe this use case to be in-spec., but I'm not sure how to do so with DJA:

Note: Full linkage ensures that included resources are related to either the primary data (which could be resource objects or resource identifier objects)or to each other.

https://jsonapi.org/format/#document-compound-documents