This trick that Rob made is a redirect to a page that can only be reached if you know the URL. So it is not hidden, there is just no link on any page in the website that would bring you to that "hidden" page.
Must a visitor be logged in with a Wikidot account to be able to see the page?
No you don't need to have a wikidot account… you could have tested this yourself by trying what rob made, but sign out first.
But if you wish to give somebody access to this page… you could better mail him the URL that would be much easier.
Also keep in mind that the page will popup in a google-search… so this is really no good way to protect something. But it is a barrier to users with no or not much internet knowledge.
So to answer your question: Does someone need a wikidot account to use this tool Rob made…
It depends on the site. If the tool is on a page of a site that requires you to be a member of the site (so a closed site) then off course this does not work. If your site is open then it will work. But remember if people can access the wikidot-source or the THML-source of your page… they will see the html block and the javascript and they could figure out the password…