This is weird because I don’t think we create any XWikiGuest user profile. So normally you shouldn’t even have a link to a profile when you’re not logged in.
IMO it’s possible that in some old versions we had a XWikiGuest by mistake and maybe when you upgraded your subwiki on myxwiki.org you kept the old page. For ex, I can see on https://massol.myxwiki.org/xwiki/bin/view/XWiki/XWikiGuest that the page doesn’t exist but that it existed in 2011 and was deleted back then.
So to fix the issue, I’d simple remove this page from your wiki.