I used vBulletin with vBseo before which regulary transfered sitemaps to Google. XenForo seems not to transfer sitemaps, says the Google webmaster tools search console.
Is it necessary to to something to activate sitemaps?
Robots.txt and sitemaps should always be in synch. In other words: if the xenforo sitemap sends a URL to search engines, then it should not be disallowed in robots.txt or it will cause crawl errors.
Conversely: if a directory is not disallowed in robots.txt, then it should not be inaccessible...