Is there a feature where you could publish an article in Help Center, but hidden, so only a direct link works to access it?
No, there's no feature where you could publish a hidden article in Help Center. Making an article accessible automatically exposes it in native search results and to search engine crawlers.
Rather, it's advisable to restrict the article's visibility in Guide to only allow certain users to view it. To restrict visibility, set the Visible to setting on the article to be only visible to a given user segment. When your agents need to provide access to an end-user, they can add the user to that user segment and share the link.