When a page is not linked with any Wikidata item, the "Add links" link below interwiki section starts the "link with page" pop-up dialog window.
However, if the page contains any local interwiki link, the "Add links" starts "Create a new item" Wikidata page instead. (Just not prefilled - the interwiki is not utilized).
I can see no logic and no benefit in such different behaviour.