Got it—thanks for confirming! Given that customers have no way to create this query themselves, I think it strengthens the case for Prismic clarifying this within their own UI. Clarifying the relationships between content (media, documents, etc.) seems like an important part of a CMS—or at the very least, one that customers can grow with.
I'll be sure to upvote posts in A few requests for Media Library and encourage others reading this to do the same: