When doing an API call using sites & titles, the result doesnt distinguish between
a) titles which do not have a corresponding wikibase_item, and
b) titles which can not have a wikibase item, e.g. the title doesnt exist, or the title are not permitted on Wikibase (talk pages)
i.e., all of the following only say 'missing'
Non existent page:
Whiteboard: u=dev c=backend p=0