Appears 'site pages' is now another storage mechanism that can't be integrated into documents.
Splitting out content by format is not how libraries should work to the end user's benefit. It would be like splitting a physical library of books about into hard-back books, soft-back books, and books that contain an optical disk in them; people trying to find all the books about Sharepoint would have to (a) know to look in three places and (b) go to each shelf separately to discover what is there.
Having a single place for content about a subject is hugely beneficial, means people only have one place to go to find or add the information.
I know there are convoluted workarounds to this - highlighted content can bring a list, site pages, and document library together into one (though users can't then filter that content unless we install a custom search webpart into the tenant). And you could have links at the top of the page to 'Add a page | Add a document | Add an external resource', however from an admin and end users perspective it's all very unintuitive.