When a page is under CUG control, then anonymous user cannot visit that page without login into application.
So when anonymous user accesses a page, that has link to secure page under CUG realm, then link-checker will disable the link since the access is not allowed for anonymous user to secure page.
So article says that - create a place holder page and put a sling:redirect property on that page that points to secure page under CUG realm. Whenever a page is accessed that has sling:redirect prop, then it ends up doing a redirect to a page that the property points to.
Check this link out for more details on sling:redirect - https://sling.apache.org/documentation/the-sling-engine/mappings-for-resource-resolution.html
Also geometrixx root node contains the same property, check that out too - localhost:4502/content/geometrixx/jcr:content
now on page that earlier contained a link to secure page under CUG realm, now you can configure the same link to point to page that contains the sling:redirect property pointing to secure page. Doing so the links will be now enabled for anonymous user and clicking of which will ask user to login.
Hope this helps :)
-- Runal